No he isn't. Read it from his own website change.gov:

from Open for Questions: Response | Change.gov: The Obama-Biden Transition Team
Q: "Will you consider legalizing marijuana so that the government can regulate it, tax it, put age limits on it, and create millions of new jobs and create a billion dollar industry right here in the U.S.?" S. Man, Denton

A: President-elect Obama is not in favor of the legalization of marijuana.
JD1stTimer Reviewed by JD1stTimer on . Obama plays down overwhelming support for Marijuana. In his response to the number one question concerning American's, only a one sentence reply was given stating "President-elect Obama is not in favor of the legalization of marijuana". Not only was the response short, it was also bumped down the line to appear as if not important. There will be another round of "Open for Questions" soon, and we should simply overwhelm them with our votes. let's make this something that cannot be ignored. Open for Questions: Response | Change.gov: The Rating: 5