The 22-year-old left-hander overcame an early blow on the helmet to strike 103 off 203 balls, with 20 fours, as his side ran up 303 for eight in reply to the visitors first innings total of 286.
Skipper Chris Dent contributed 65 to an opening stand of 182 before James Bracey (34) and Gareth Roderick (48 not out) built on the efforts of the openers.
Jofra Archer then put a different complexion on the game by claiming three wickets in the penultimate over.
Sussex's bowling was far from toothless earlier in the day, Archer and Ollie Robinson, in particular, producing some pacey spells on a pitch still offering bounce and going past the bat on numerous occasions.
Hammond and Dent had to survive a torrid first hour, which saw the latter dropped by Phil Salt at third slip off Robinson, a straightforward chance, with his score on 14 and the total 44.
It proved a costly error. Before adding to his overnight 27, the unflappable Hammond had needed a few minutes to recover, having been struck by a short ball from Robinson.
Hammond moved to his half-century off 86 balls and when the stand with Dent reached 115 it was Gloucestershire's highest opening partnership
of the season.
Dent went to 50 off 151 balls, with six fours, having dropped anchor responsibly in the face of some testing seam bowling. By lunch he and Hammond had taken the total to 137.
They were eventually parted when Dent lost concentration and helped a delivery from left-arm spinner Danny Briggs, scooping a catch to Archer and short fine-leg.
Hammond reached three figures with a square cut boundary off David Weiss, but two balls later edged a wide one from the same bowler through to wicketkeeper Ben Brown.
Roderick and Bracey then added 65 either side of tea before Bracey fell to the second new ball, lbw to Chris Jordan, trapped in his crease, with the total of 259.
Weiss bowled Graeme van Buuren on the back foot for 11 before Archer gained reward for his earlier labours by sending back nightwatchman George Drissell and Ryan Higgins with successive balls and having Kieran Noema-Barnett caught behind two deliveries later.
When Robinson pinned Craig Miles lbw in the final over, Gloucestershire had lost five wickets for eight runs to surrender much of the initiative.
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