Chelsea’s poor spell continued as they dropped more points and produced another disappointing performance.
Hakim Ziyech put them ahead with a goal out of nothing shortly before the half-hour mark.
Ziyech collected N’Golo Kante’s lay-off and tried his luck with a long-range effort which caught out keeper Robert Sanchez and ended up in the bottom corner of the net.
But Brighton caused the Blues problems, especially in the second half, and got the equaliser they deserved.
Keeper Kepa Arrizabalaga did well to push away Alexis Mac Allister’s shot but could do nothing about Adam Webster’s thumping header from the resulting corner.
Chelsea, now without a win in four matches, remain third in the Premier League, a point behind Liverpool and 12 points behind leaders Manchester City.
Chelsea: Kepa, Azpilicueta, Silva, Rudiger, Hudson-Odoi, Kante, Jorginho (Kovacic 81), Mount, Alonso, Ziyech (Werner 81), Lukaku (Havertz 81).
Subs not used: Bettinelli, Pulisic, Loftus-Cheek, Niguez, Sarr, Hall.