Japan was a member of the Entente powers during the First World War, particularly as an ally of convenience with Great Britain.

Let me explain.

When the First World War broke out and just before Great Britain was drawn into the conflict upon the German invasion of Belgium, the Royal Navy, as big and powerful as it was, found itself stretched rather thin and unable to adequately commit to a naval blockade of the North Sea, the protection of lines of communication, the escorting of convoys of troop transports such as those that would sail from Australia for Egypt and the pursuit and destruction of German surface raiders operating in far off waters such as the Pacific and Indian Oceans. Japan had enjoyed more then a decade as an ally of Great Britain since the Anglo-Japanese Alliance was signed in 1902 and renewed in 1905 and in 1911. While Britain was hesitant to embolden Japanese military activities in the Pacific by asking the Japanese government to assist (and knowing the diplomatic issues it would cause) with naval protection duties, there was really no alternative as the British dominions in the region lacked the naval power required and the United States was not willing to get involved.

On 7 Auguest 1914, Ambassador Inoue, the Japanese ambassador to London, received the following from the British Government:

(the British government) would gladly avail themselves proffered assistance of Japanese Government in the direction of protecting British trading vessels from German armed merchant cruisers which are now attacking British commerce in various quarters, while British war ships are locating and engaging German war ships in Chinese waters. As this task will require some time, H.M. ambassador in Tokio has been instructed to inform Japanese Government that it would be of the very greatest assistance to H.M. Government if they would be good enough to employ some of their war ships in hunting out and destroying German armed merchantmen in China. British Government realize that such action on the part of Japan will constitute declaration of war with germany, but it is difficult to see how such a step is to be avoided.

The above communication was intended to restrict any Japanese involvement strictly to the protection of British trade and the destruction of German surface raiding vessels. Such a proposal offered no benefit to Japan and when the proposal was discussed in Tokyo, it was decided that if the country was to enter into the conflict, Japan's naval operations should not be restricted and that instead should be used to develop Japan's role as a regional power. It was ultimately decided that Japan would issue an ultimatum to Germany that would demand Germany transfer the leased territory of Tsingtao (a Chinese territory held by Germany since 1898) to Japan. Japan rationalised this demand to Britain by explaining that in order to secure its security in the region and to prevent its use as a safe harbour for German vessels, the port would have to be seized. Germany refused the ultimatum and on 23 August, the Japanese Navy blocked the port. On 2 September, troops were landed, eventually numbering some 30,000 men. In comparison, Tsingtao had a garrison of 3500 men. Resistance was relatively short lived with the port and surrounding areas being captured by early to mid-November.

Meanwhile the Japanese navy was underway in the waters of the Pacific, seeking out German commerce raiders and watching out for the German China Squadron consisting of two cruisers, the Scharnhorst and the Gneisenau, the cruiser Emden, the Leipzig and the Nurnberg. This was a considerable force and posed a serious risk to the Allies in the region and actually led to the postponmenmt of Australian and New Zealand troop convoys due to the danger they posed and the inability of the British to provide an adequate escort. Ultimately it would Japan that would lend the battle cruiser Ibuki to the British for escort duty, an undertaking that was not received well by the dominion governments.

At home in Japan, public opinion was demanding that the country receive some compensation for undertaking the tasks requested of it by the British. Ambassador Kato presented this issue to the British and stated that such compensation would come in the form of Germany's Pacific possessions which were in the process of being seized by Australian, New Zealand and Japan. No request for the transfer of any islands should be made by the British to the Japanese for any islands or possessions captured by the Japanese. Between Septemeber and October, Japanese naval forces captured the Marshalls, the Carolines and the Marianas island chains, all of which had previously been occupied by the Germans and used for communications and coaling of naval vessels. The capture of these islands was of particlar concern to Australia which has long possessed a fear of first Chinese and then Japanese expansionism into the region and the risk of invasion. Of particular concern to Australia was the island of Yap in the Carolines which served as a major German communications centre. The Australian government had proposed to the British to send an expeditionary force to Yap and takeover the occupation of the island from the Japanese. The Japanese government agreed to this but the navy protested, leading to a diplomatic crisis between Australia, Britain and Japan.

Due to the ever present fear of Japan that had gripped Australia since the first decades of colonisation, the possibility of Japanese expansion into the Pacific was a wholly terrifying prospect. When the Japanese refused to transfer fromer German possessions to Australia, the Australian government, then under Prime Minister Andrew Fisher, protested to the British government. The British government was placed in a difficult position. on the one hand it had a duty to protect and assist its dominions but on the other it required continued Japanese naval assistance to seek out and destroy the German naval forces still operating in the region. Ultimately, the British opted in favour of the Japanese and it was communicated to Fisher that Japan would retain control over the islands until the war was concluded and the future occupation of the isalnds would be decided at the post-war peace conference. Secretly though, the Australian governor-general, Sir Ronald Munro Ferguson was told by a member of the british government that he best prepare government ministers for the likelihood that Japan would retain control over the islands even after the war. When Fisher resigned and was replaced by William Hughes, Munro Ferguson informed the new Prime Minister of such a likelihood, Hughes, who was the very embodiment of Australia's fears of Japan, set out to Britain to discuss the matter. Finding no satisfaction, he returned, announcing that Australia needed to introduce conscription to maintain its fighting forces in France. My current thesis is exploring how much the issue of Japan influenced Hughes' decision to pursue conscription and argues that Hughes saw conscription as a necessary to tool to ensure Australia maintained a major role in the outcome and so secure a louder voice at the post war peace conference where Australia security could be ensured.

Sources:

The Anglo-Japanese Alliance and the First World War by Yoichi Hirama

Alliance in Decline by Ian H. Nish

Australia and World Crisis 1914 - 1923 by Neville Meaney

Search for Security in the Pacific 1901 - 1914 by Neville Meaney

Quite a fair bit actually. The Japanese Empire during the First World War entered the side of the Entente Powers on the 23rd of August 1914, just 19 days after their British ally declared war on the German Empire. If you are capable of reading Japanese (or know someone who does), here is a scan of the actual declaration of war issued by the Japanese government. Although tertiary sources often downplay the role of the Japanese Army and Navy in the war, it is important to note that the "Ally from the East" played an important role in the entire Allied war effort.

Firstly however, it is necessary to understand why Japan entered the First World War. It seems almost curious that a Far East Asian power embroil itself in a conflict occurring on the other side of the world amongst (at least until 1917 and the entry of America), exclusively European belligerents. However, Japan had signed the Anglo-Japanese Alliance in 1902 with Great Britain, but this treaty did not actually bound Japan to entering the war on the side of its British ally. What it did lead to however, was the ability of the British to appeal to Japan for aid in its own war effort. This it did, and on the 7th of August 1914 (just three days after London declared war on Germany), it formally requested that Japan assist in the identification of hostile ships in the Indian Ocean and Southwest Pacific. More specifically, the feared ships of the Ostasiengeschwader, or East Asia Squadron of the Kaiserliche Marine (Imperial German Navy), who were based out of the Kiautschou Bay Concession in China. Such an act was not a neutral one, and the British admitted as such in their request. The Japanese government however, eager to secure more imperial possessions and establish Japan's place as a true world power which matched (or even surpassed) their European counterparts, formally joined the war on the side of the Entente.

The Imperial Japanese Navy and Army went off to battle almost immediately, with their main campaign being against German Tsingtao (or Qingdao), a concession territory on the Shantung (or Shandong) Peninsula in China. From August to November 1914, an Anglo-Japanese force besieged the city from land and sea, blockading the tiny possession from any reinforcement. The German and Austro-Hungarian defenders formally surrendered on November 7th that year. Notably, all modern (and major) German naval units in Tsingtao had put to sea just before the arrival of the Anglo-Japanese forces, chief among them the ships of Vizeadmiral Maximillian von Spee's Ostasiengeschwader.

Elements of the Imperial Japanese Navy, namely the Second Fleet, assisted their British, French, and Australian counterparts in sweeping the Pacific for signs of the East Asia Squadron. The battleship Kongo was dispatched to Midway to patrol the sea lines of communication, whilst the cruiser Izumo safeguarded Allied shipping off the coast of Mexico. battle cruiser Ibuki and cruiser Chikuma assisted other Allied naval elements near Singapore, and Chikuma operated as far as the Bay of Bengal. The battleship Satsuma, alongside cruisers Yahagi and Hirado, patrolled the sea routes to Australia in the search for German raiders. On the 16th of October, Ibuki undertook a new role which would become a crucial element of Japan's naval involvement in the First World War: escorting ANZAC troops to the Middle East theatre. Japanese ships would escort the convoys as far as Aden on the Red Sea, and some would escort French convoys carrying troops from Indochina to the Red Sea.

In late 1914 however, Australia and New Zealand grew concerned with the actions of the Japanese Navy, when ships of its First Fleet seized the German colonies of the Marshall, Caroline, and Mariana Islands. These seizures had not been previously approved by the British government, but London was unwilling to anger its crucial ally in the Pacific. It was soon agreed that the Empire (that is, Australia, New Zealand, and Britain) would not seize any more German territory north of the equator. The Japanese Navy would practically control these island holdings until the civilian government formally took over administration in 1921 (something which increased the level of influence the Navy held over the government, and was a sign of things to come in the 1930s.)

Perhaps the most forgotten aspect of Japan help in the First World War however, was the dispatching of the Mediterranean flotilla. On the 11th of March 1917, Admiral Sato Kozo set out from Singapore with the Tenth and Eleventh Destroyer Flotillas, which consisted of the cruiser Akashi alongside the destroyers Ume, Kusunoki, Kaede, Katsura, Kashiwa, Matsu, Sugi, and Sakaki. These ships were crucial in safeguarding the flow of Allied shipping throughout the Mediterranean, where Austro-Hungarian mines and submarines (German or Austro-Hungarian) wreaked havoc. Reinforced throughout 1917 (including the temporary manning of two British gunboats by Japanese sailors), the Mediterranean flotilla at peak strength consisted of 17 ships flying the flag of the Rising Sun. Their performance won them praise from the British admirals who were present in the region, and they held the highest ratio of time spent at sea to time spent at port (72%, the British managed 60%, the French and Greek 45%). On the 21st of August 1917, Admiral George A. Ballard (Senior Naval Officer-in-Charge at Malta), dispelled any concerns the Admiralty had that the Japanese were not up to the task:

"French standards of efficiency are certainly lower than the British, however and Italian standards are lower still. With the Japanese it is otherwise. Admiral Sato's destroyers are kept in a highly serviceable condition and spend at least a large proportion of their time at sea as their own, which is far from being the case with the French and Italian vessels of any class. The Japanese moreover are very independent in all manners of administration and supply whereas the French will never do anything for themselves if they can get it done for them."

Note: I find it somewhat amusing that even in the Great War, the officials of the Royal Navy have no qualms against absolutely destroying their French counterparts, even if they were technically allies.

Beyond the Imperial Japanese Navy's indispensable contribution to the Allied naval operations in the Pacific and Mediterranean, the Japanese later assisted with the Siberian Expedition, dispatching some 70,000 troops to assist the Allied intervention in the Far East of Russia (which was of course, in the midst of a civil war at the time). Politically, they also took the opportunity to present the 21 Demands to China in 1915, which essentially required the Chinese to accept Japanese dominance and influence over their affairs. The United States and Britain did not support this policy, and the Japanese government, although an ultimatum with "Thirteen Demands" (the "Group 5" demands had been removed, as these were the ones which would essentially make China a Japanese protectorate) was accepted by President of the Republic of China Yuan Shikai.

Hope this response helps answer your question, and feel free to enquire more on Japan during the First World War as you see fit.