Plesac, 41, made 58 appearances for the Phillies last season, going 2-1 with a 2.70 ERA. He was the last Phillies' pitcher to record an out at Veterans Stadium, striking out Ryan Langerhans of the Atlanta Braves.
"Dan did an outstanding job for us in our bullpen in both 2002 and 2003 and we were looking forward to having him back in 2004," said General Manager Ed Wade. "Dan has talked about retirement a number of times, and this time he says he's serious about it."
Plesac ranks fourth on the all-time appearance list with 1,064, passing Lee Smith and Kent Tekulve last season. He needed only six more appearances to tie Hoyt Wilhelm for third.
Plesac had a career record of 65-71 with 158 saves and a 3.64 ERA.