Dennis J. Sullivan (June 26, 1858 – December 31, 1925) was an American utility player in Major League Baseball, playing mainly as a third baseman for the Providence Grays (1879) and Boston Red Caps (1880) of the National League. A native of Boston, Massachusetts, Sullivan attended Boston College and College of the Holy Cross. Listed at 5 ft 9 in (1.75 m), 170 lb., he threw right-handed (unknown batting side).

Denny Sullivan
Third baseman
Born: (1858-06-26)June 26, 1858 Boston, Massachusetts
Died: December 31, 1925(1925-12-31) (aged 67) Revere, Massachusetts
Batted: Unknown Threw: Right
MLB debut
August 25, 1879, for the Providence Grays
Last MLB appearance
September 25, 1880, for the Boston Red Caps
MLB statistics
Batting average	.261
Home runs	0
Runs batted in	3
Teams
Providence Grays (1879) Boston Red Caps (1880)

In a six-game career, Sullivan was a .261 hitter (6-for-23) with six runs, three RBI, and two doubles without home runs. He made six appearances as a third baseman (4), catcher (1) and left fielder (1), and was a member of the Providence 1879 National League champions.

Sullivan died at the age of 67 in his homeland of Boston, Massachusetts.