Is it true that Karl Marx was a Freemason?

It is difficult to say for sure. Marx never claimed to be one, and talked about Freemasons as if he did not know (or care) much about them. The sources which suggest that he was a member of a lodge do name a real Lodge attached to the Grand Orient of France in Osterbruck (L'Étoile Anséatique), but Marx had no connection with Osterbruck and it is highly unlikely that he would have joined a lodge there.