Obama's fundraising haul topped the more than $114 million raised by his campaign and Democrats during the month of August. Republican presidential nominee Mitt Romney's campaign and Republicans have not yet reported September fundraising totals.
Obama says in a message on the online social network Twitter that more than 1.8 million people donated in September, including more than a half-million who had not donated before in 2008 or 2012.
The $181 million was slightly less than Obama's record of about $190 million in September 2008.
Obama's campaign ended August with nearly $89 million in the bank. It did not release its current cash on hand.