Kazenga LuaLua hit a last-gasp leveller to deny Millwall a third away win in four starts in a ding-dong battle at the Amex Stadium.
Millwall's first ever visit to Brighton's new ground looked as if it was going to end in victory for Kenny Jackett's charges as Liam Feeney (64mins) put The Lions within touching distance of three points after Sam Vokes (52mins) had cancelled out Andy Keogh's 23rd-minute opener.
But sub LuaLua, who had only been on the field for three minutes, drilled home a low shot from 18 yards following a well-worked set piece to secure a share of the spoils for Gus Poyet's high-flyers.
On paper this looked like a good solid away point for Millwall, but at the final whistle it felt more like a defeat as Jackett's men failed to take the maximum points their fluent display deserved.
Millwall showed three changes to the team that started against Derby County on Saturday with Jack Smith, Liam Feeney and Keogh replacing Josh Wright, James Henry and Harry Kane, who dropped to the bench.
Backed by a vociferous away support of around 2,500 The Lions had a measured, composed look about their play, much to the frustration of the home support.
A neat passage of play on 10 minutes saw possession switch from Scott Barron on the left to Feeney on the right, the latter's deep delivery was chested down by top-scorer Darius Henderson but no-one was there to get on the end of the loose ball.
The Lions went close again in the 12th-minute when Keogh's fierce drive from a Barron corner was deflected past the post by the outstretched boot of Gordon Greer.
At the other end Millwall were perhaps guilty of trying to over-complicate matters when clearing a Brighton attack, enabling Inigo Calderon the chance to pick out Vokes, and his first time volley flew a foot wide.
However, it was Millwall who broke the deadlock after 23 minutes when home keeper Peter Brezovan punched clear Barron's corner, Alan Dunne headed back across goal and Keogh planted a firm header into the back of the net.
Brighton responded and Forde made a smart stop to deny Ashley Barnes, but then nearly gifted the home side an equaliser when he spilled another Barnes shot straight into the path of Vokes, who somehow screwed wide with the goal gaping.
Skipper Paul Robinson then threw his body to brilliantly block from Alan Navarro as Brighton threatened. But back came The Lions and Trotter went close to doubling Millwall's tally within 60 seconds of the restart when his deft footwork took him past a couple of Albion defenders before seeing his shot well saved by Brezovan.
Two minutes later Keogh was guilty of firing wide from close range when he really should have done better, and that proved to be a costly miss as Poyet's men hauled themselves level.
Navarro picked up the ball around 25 yards out and although his initial shot was blocked, the ball fell kindly for Vokes, who slammed home from eight yards.
It was a cruel blow for The Lions, who were left breathing a sigh of relief when Vokes broke into the danger zone again, but this time poked the ball straight at Forde.
However, it was The Lions who got their noses in front just past the hour mark when great work by Keogh on the left set up Feeney, who stroked the ball high into the roof of the net at the far post.
And Trotter, who was immense from start to finish, so nearly added what would surely have been a decisive third goal just moments later when he picked his way through the home defence with a superb run, but curled the ball agonisingly wide with Brezovan grasping at thin air.
Henderson was next to test the home keeper with a fierce angled drive following a delightful chipped pass by Trotter as The Lions threatened again.
Vokes saw his 86th-minute shot bobble wide of the target as Brighton pushed forward in the dying stages in search of a late leveller.
And sub LuaLua duly obliged, rifling home from a central position with a powerful drive that whistled through the Lions' wall and hands of keeper Forde on its way into the back of the net.
Even then, Millwall had an opportunity to snatch victory in injury-time when Feeney's deep cross from the right picked out Trotter, who unfortunately steered his header a yard wide of the target.
Harry Kane floated a 20-yard free-kick over the bar with the last kick of the game as Millwall so nearly secured the third, killer, goal that would have clinched three points instead of just the one.
Brighton & HA: Brezovan, Mattock, Greer, Dunk, Navarro, Barnes, Mackail-Smith (Noone, 56mins), Calderon, Vokes, Bridcutt (LuaLua, 85mins), Reyes (Agdestein, 56mins). Subs not used: Ankergren, El-Abd.
Bookings: Barnes (53mins), Greer (90+3mins)
Scorer: Vokes (52mins), LuaLua (88mins)
Millwall: Forde, Dunne, Barron, Smith, Robinson, Lowry, Mason (Wright, 62mins), Trotter, Keogh, Henderson (Kane, 73mins), Feeney. Subs not used: Mildenhall, Henry, N'Guessan.
Bookings: Mason (6mins), Forde (79mins)
Scorers: Keogh (23mins), Feeney (64mins)
Ref: A. Taylor
Att: 19,503 (away 2,357)