The 24-year-old New Zealander Phillips led a superb recovery from 35 for four at the end of the powerplay to 162 for five against the previously unbeaten Sharks.
Gloucestershire’s seamers, led by Benny Howell who took four wickets, then did their bit to ensure Phillips’ efforts did not go to waste. Only Luke Wright (33) got the measure of conditions as Sussex were dismissed for 135.
Such a scenario looked unlikely when Gloucestershire made a poor start having been put in. Robinson had Miles Hammond (3) caught at long on off his third ball before Howell (4) was run out by Ravi Bopara’s direct hit from extra cover.
Robinson, playing again after the brief break from cricket he took after his superb England Test debut was overshadowed by the emergence of racist and sexist tweets he posted when he was 18, got a warm reception from a Sussex crowd who are glad to see him in action again for them and who will be hoping he gets another England chance soon.
Tymal Mills had Chris Dent (16) caught behind and James Bracey (1) taken at mid-wicket in his first over but that was the last success Sussex enjoyed until Mills, who finished with three for 20, removed skipper Jack Taylor in the final over.
Taylor helped Phillips add 124, a new Gloucestershire T20 record for any wicket against Sussex, although it wasn’t until deep in the innings before they really cut loose when 82 runs were plundered in the final seven overs, including 25 off left-armer George Garton in his last appearance before he joins up with England’s one-day squad.
Phillips, who made an unbeaten 94 against Glamorgan on Thursday, didn’t hit his first six until the 12th over but added four more and seven boundaries, to take his competition aggregate to 266 runs while Taylor gave him excellent support with 38.
Sussex would still have fancied their chances of chasing at eight an over, but they never recovered from losing Phil Salt (11) and Travis Head (1) in David Payne’s opening burst.
Bopara (4) and David Wiese (6) also went cheaply and after skipper Luke Wright sliced a drive off Benny Howell to backward point for 33 in the ninth over Sussex’s race was effectively run.
Howell bowled outstandingly, finishing with four for 15 as the Sharks slumped to 78 for eight before Robinson (31) and Mills (27) hit career bests at the end to provide the 2,500 crowd with some entertainment.
Mills said: "It was a disappointing loss. After winning our first three and putting in good performances I think it showed that we hadn't played for three games, apart from five overs against Surrey. But we should still have chased that total. It was a used wicket and perhaps we needed to adapt better to it. We're still in a good position - it was our first defeat - and we have to roll on against Surrey on Sunday."