First off: the death of Jan Masaryk is a true mystery. It has been investigated by the Czechoslovak/Czech police multiple times, and most recently reopened in 2019. The results, which were published in March 2021, were inconclusive as to whether it was murder, suicide, or an accident. You probably already know this but I wanted to emphasize just how very mysterious this all is, because in basically everything I've come across that discusses Masaryk's death in English, there's an insinuation that "died under suspicious circumstances" really means "of course he was killed by the Communists but there's no smoking gun", the assumption being that the Czechoslovak Communists obviously benefited from his death.

No, there is no clear evidence that the leadership of the Czechoslovak Communist Party (or the KSČ -- I'm just going to use this acronym from here on because it's easier) ordered Masaryk's murder. That doesn't mean they didn't, but it is not considered very likely. Nobody expects to find something as obvious as a resolution in the minutes of the Central Committee to push the Minister of Foreign Affairs out a tiny window, but there is no shortage of documentary evidence for all kinds of other crimes and skullduggery on the part of the KSČ and the police. Historians have thoroughly investigated what exactly the KSČ leadership was doing in February and March of 1948. Many of the officials who would have been involved in any murderous orders issued from the top were purged or fell out of favor, so it's not as if there would be no opportunity for anyone to say anything later. More importantly, the KSČ's motive for killing him is much less clear than it might seem at first.

Masaryk was the only really independent government minister who didn't resign in late February. In fact, shortly after the new Communist-dominated government was confirmed, he declared "I'll be happy to govern with this government." The KSČ certainly would have preferred to have one of their own members as foreign minister, so it's hard to believe he'd remain in that position for too long. But in March 1948, Masaryk's cooperation legitimized the the KSČ's seizure of power. If he stopped cooperating, the KSČ leadership could have been confident in their ability to sideline him without too much trouble. Granted, if he emigrated and started rallying opposition from abroad, this wouldn't be great, but it's also not great when a popular government minister who also happens to be the son of the beloved founder of your country dies under suspicious circumstances just a few weeks after your party seizes power. Revolutionary violence and assassinations were completely counterproductive for the image the KSČ was trying to project. Their line on the "February events" (as they referred to their seizure of power) was that they did not represent a coup or a revolution, but rather a defensive move that was necessary to preserve the revolution that had already happened in 1945. In fact, according to them, they had averted the coup that the non-Communist ministers were plotting. This isn't to say that they were somehow above brutality or repression -- clearly not -- but that there are different ways to try and gain a monopoly on power.

The best sort of circumstantial evidence for the KSČ's involvement in Masaryk's murder is how the Czechoslovak secret police treated the investigation. The KSČ, which had been in charge of the Interior Ministry since 1945, absolutely used the police for their political purposes. In fact, this is what sparked the government crisis in February 1948. So it's fair to look at the actions of the police and draw conclusions from that about the intentions of the KSČ, and it's fair to say that the police basically decided Masaryk had to have committed suicide and set out to prove that. Also, police officers and investigators did dumb things that destroyed potential evidence (things like letting people walk through the apartment and leave footprints everywhere -- I don't know enough about how police investigations were conducted in Czechoslovakia or anywhere else during the 1940s to say how unusual this was, but in retrospect it looks very unprofessional). We know a fair amount about how the investigation was conducted, because a number of the people involved were interviewed in 1968. But what the dodgy 1948 investigation indicates is not necessarily that the KSČ ordered his assassination, but rather that they believed that a full investigation of his death would be politically damaging for them.

I hope this answers the question. I interpreted it as literally as possible -- to refer specifically to the KSČ as an organization, rather than to Communists in general. There were a number of shadowy figures with KSČ connections lurking around in high places in Czechoslovakia at the time, but to address them would require a certain amount of speculation that this isn't the place for. I also didn't address the possible involvement of foreign intelligence, although obviously this is very significant in any discussion of who might be responsible for Masaryk's death.