Brown says in announcing his veto Sunday that he is "troubled by the fact that some family law specialists believe the bill's ambiguities may have unintended consequences."
SB1476 would have allowed judges to legally recognize multiple parents when it is in a child's best interest.
Democratic Sen. Mark Leno, of San Francisco, says his bill recognized that times and families have changed.
Supporters say it could have kept children out of foster care by giving them additional legal parents who have caretaking and financial obligations.
Opponents say the bill harms traditional parental roles and could allow children to have a limitless number of parents.
