The former England batsman, 31, hit 16 fours in his score of 119 to anchor the hosts' innings alongside Darren Stevens (44) and Will Gidman (42).
Vernon Philander (2-42) and Jofra Archer began well, reducing Kent to 115-4 before the middle order rallied.
David Wiese (3-54) removed James Harris late on, but Adam Rouse reached stumps 32 not out to conclude the opening day.
Philander and Archer piled on the pressure early on, restricting Kent to 7-1 from their first 10 overs before Wiese struck to leave the home side in trouble.
However, Denly frustrated Sussex in his 208-ball innings, finding support from Joe Weatherley (33), Stevens and Gidman before Philander pinned him lbw.
Joe Denly struck his 20th first-class century to help Kent reach 316-8 on day one against Sussex at Tunbridge Wells.