What exactly are you asking? Your question is unclear because it's not obvious what you mean by 'assassination' As Merriam Webster defines it, the term 'assassination' means:

murder by sudden or secret attack often for political reasons : the act or an instance of assassinating someone (such as a prominent political leader)

It is politically-motivated murder, not least of prominent political leaders (which he certainly was). The term does not mean a conspiracy has to be involved; plenty of assassins have acted alone throughout history.

So if you're really asking if he was murdered for political motives, then yes, that's always been the main theory. Palme was not killed in an argument, fight or physical altercation. He wasn't being mugged. By the accounts of the main eyewitness Lisbet Palme, a man just walked up and shot him unprovoked.

As the murder remains unsolved we cannot truly know the killers motives. But if the crime had a motive, the political one is the most obvious one, perhaps the only obvious one. There are many conspiracy theories about who killed him and why, of all ranges of plausibility, but that's another matter.

Or to put it simply: The murder of Olof Palme is generally referred to as an assassination.

(it is however referred to simply as a 'murder' in Swedish, as there's no entirely equivalent term to 'assassination'. The term 'lönnmord' (literally: "secret murder") is closest but is used mainly to refer to assassinations done more covertly, such as with poison, not out in the open like this)