Rory Hamilton-Brown starred with both bat and ball at Hove as Sussex condemned holders Middlesex to their fourth Twenty20 defeat of the season.
After taking 3-15 to help bowl Middlesex out for just 116, Hamilton-Brown struck one six and 10 fours on his way to a 47-ball 69 not out.
This took Sussex to their target inside 17 overs for loss of three wickets.
Middlesex remain rooted to the bottom of the South Division without a win in four games.
Having won the toss and elected to bat only Gareth Berg (33), Tyron Henderson (20) and Shaun Udal (20) made any real impact for the visitors.
Michael Yardy pitched in with 3-22 as Sussex moved to third in the table.