Residents of Los Angeles' most famous neighbourhood woke up on New Year's Day to find the world-famous Hollywood sign had been changed to read "Hollyweed".
Local media reported that police were treating the incident as minor trespass and were investigating.
The sign on Mount Lee is made of 45-foot (13.7m) tall letters.
Voters in California approved the legalisation of marijuana in a ballot held at the same time as the presidential election - on 8 November.
The prank has not caused lasting damage to the sign, however, as parts of both "O" letters were covered by tarpaulins to make them look like a lower-case letter "E".
The Los Angeles Times reports that a single person was recorded on security cameras climbing the sign to hang the materials.
A similar prank took place in 1976, to mark a relaxation in the state's marijuana laws.