"Depending on how broadly you want to define shield, samurai might have used shields more than many think.

These were used fairly routinely throughout the centuries of Japanese warfare, even as other kinds of shields were phased out. They were used a great deal during the Sengoku Era, especially during sieges and even more so once firearms were introduced.

That being said these pavise were definitely not as ubiquitous as the run of the mill shield in other parts of the world, even as close as China.

This is due to two major differences in the way Japanese warfare evolved as we begin to enter the era when samurai were most prominent, which is starting in the 11th - 12th century all the way up to the end of the 16th and very beginning of the 17th century, which by then the Edo Period had been established and nation wide peace reigned for hundreds of years.

It is due to the difference with their weapons and the change in organization during combat.

At the beginning, we have the Heian Period. During this time, battles and warfare were nearly always fought between very small, dedicated forces who followed protocols and rather peculiar rules of combat. The procedures for engaging in combat were fairly ritualized.

This is in stark contrast to the crush and brutality of battle during the Sengoku Era in the 16th century.

It is here that we begin to see the pronounced trending toward individual combat as opposed to a focus on ranks and formations. This partly has to do with the great emphasis on personal strength and skill within both the warrior culture as well as incentive in promotion and climbing the social ladder.

It's much easier to show everyone (and to prove) what a badass warrior you are when you don't have to compete for the kill with 20 other men.

As we move on into the Sengoku Era, war has become much more brutal, widespread, and all consuming. It's not longer just the professionals who go to war, but all the able bodied men as well.

This is where organized formations become important again, because of the massive influx of recruits and attempts by commanders to maintain their tenuous control on chaotic battles. But even then, their formations and organized tactics do not reach the level of precision that one would see in a well drilled phalanx.

Japanese military doctrine during this time still placed a great amount of confidence in the strength of the individual. In a sense, the Japanese military thinkers see formations as a tool for those who cannot manage themselves in a fight without strength of numbers. The best warriors, the elites, will always engage in single combat, where they can draw out their full potential.

Now, this doesn't mean that they didn't help one another or use tactics that utilized advantage of numbers when an opportunity presented itself. If 3 red guys corner a loner from blue, the red guys won't just leave because they want their own dancing partners.

But in general, Japanese soldiers would not advance in lockstep and fight as a cohesive unit once battle was joined.

They would go out in search of an opponent(s) and simply duel it out. Shields are arguably most effective when used jointly in a solid, well drilled formation.

The other reason is the style in which they used their weapons as well as the weapons themselves.

KingofAlba has done a great job in covering the reasons why a shield would not be handy.

Spears were the primary weapon for the samurai in melee, not the katana.

Even assuming they used the katana, it's extremely unwieldy to use the average katana in a single hand. Now this is not about the weight, exactly. Katana weighed in at aprox .7 ~ 1.5 kg across the board, basically exactly the same as any other medieval sword.

The key here is the balance of the katana. Most were made with a two hand grip in mind, and the balance was done accordingly.

Soldiers would have been taught to use their katana in a two hand grip and when striking the movement they used primarily was a slashing motion that pulled back towards the body, aimed at gaps and weaknesses in the armor. This sort of cutting was the most efficient attack with a katana and a shield would most certainly get in the way of that.

As for their spears, they were also taught to use them in a two hand grip. Those that used the longer yari would have most likely engaging in a spear wall. Shorter yari were used on horseback and shorter still would have been for close, individual combat. These would have been used in a two hand grip as well.

The style of attack with a yari would have involved a great deal of leaping and rolling around, parry, close, and strike. The huge range of motion is probably key here. A large shield would only get in the way of the Japanese style of spear fighting.

I guess the thing to take away from this is that a shield was seen as obtrusive and got in the way of moving around freely for the style of fighting that the Japanese did, particularly with spears.

With the adoption of firearms, pavises on the battlefield came back into style in a big way but hand held shields were still not considered anymore useful or viable."

This is a post from /u/AsiaExpert that should hopefully answer your question, as it was on essentially the exact same question. It covers a period of time more broadly than the warring states period (Sengoku Era), but covers that as well.

If you check our FAQ you'll come across some answers to this question already.

Although if anyone has more to add then let's hear it...