There is no evidence at all. The whole issue began when, at the 400th anniversary of Columbus's discovery of America (in 1892), a Jewish organization in the US charged scholar Moritz Kayserling to investigate if there was any role of Jewish in the discovery of America. Kayserling investigated, then published the speculation that Columbus might have been Jewish, based on some pretty vague pieces of information and pretty thin coincidences. At the time (late 1800s), there had been a wave of Jewish immigration, and some resentments about Jews being outsiders arose in the US. Same situation for other nationalities, such as Italians. By tying their group in with someone so quintessentially important to American history as Christopher Columbus, it was a way of saying that Jews belong in America, too. Likewise, Italian immigrants played up the importance of Columbus Day and began celebrating it as a day to celebrate Italian heritage, much as Irish-Americans began celebrating St. Patrick's Day in the US (which at the time was not a big holiday in Ireland).

It's true that one of the few things we know about Columbus is that he was Italian. But there's no evidence he was Jewish, and there was never a hint or a whisper that he might be Jewish until the 1892 article, despite people of the 1400s having ample opportunity to discuss the issue. It's true that the year 1492 was the year all Jews were expelled from Spain. Specifically, they had to be out by July 31. Columbus sailed August 3rd. That is no kind of evidence that Columbus was Jewish. It was just a favorable time to be sailing. Had he been Jewish, Ferdinand and Isabella never would have supported a venture with him at the helm. Had he been Jewish yet been concealing it, then he would have had no reason to leave Spain at that time since nobody would have known the law applied to him. He did have a few crew members who were Jewish, though, so it was a good time to leave for them, and it's always possible he arranged his departure to be convenient to obtain some crew members. And remember, Columbus was Italian, so he had no reason to need to flee Spain even if he were Jewish--he was not a refugee. After the Americas were discovered, Ferdinand and Isabella issued a decree not allowing Jews to go there. So why would they support Columbus going there?

Also important: if you study the history of the treatment of the Jews of Spain at that time, you'll learn that of great concern at that time--in fact, it was the reason for the Spanish Inquisition AND the expulsion--was the belief that there were lots of "crypto-Jews". These were people who were secretly Jewish who went around pretending to be Christian in order to get the benefits of being a Christian (such as being able to hold certain jobs, go to certain schools, live in certain places, and be accepted by chic society) while still being Jewish in the secrecy of their own home. This was a very very serious crime, one punishable by death at that time and place. It was heresy to be a Christian but to secretly practice a different religion. So had Columbus been deceptive in this way, it would have been a BIG DEAL, and vastly more dangerous than just being Jewish openly. A main reason for the explusion was that Ferdinand and Isabella felt that if all Jews were expelled, then the hatred and violence being directed at the alleged crypto-Jews would go away, since the crypto-Jews would no longer have Jewish friends, family, and neighbors to consort with. (In reality it didn't help, since the crypto-Jewish accusations were largely a witch hunt).

Check out the information in Modern Judaism and Historical Consciousness: Identities, Encounters. Edited by Andreas Gotzmann, Christian Wiese