Ramnaresh Sarwan hit a brilliant, unbeaten 104 as West Indies amassed 261 for six from their 50 overs.
But Trescothick got the tourists off to a flying start before Andrew Strauss hit 66 as England reached 262 for five with 16 balls to spare.
Paul Collingwood, with 46 not out and Chris Read, with an unbeaten 23, saw England home.
"This was an important win for us," said England captain Michael Vaughan. "Knowing we were on a flight home tomorrow (Thursday), it would have been easy to just fold but you saw how the players were in the field today.
"That shows the work ethic and the mental strength of all the players. They have been like that all through the tour and it's been a fantastic couple of months."
Lara admitted that his team had lost the match in the field, conceding 111 runs in the first 15 overs.
"From the position we were in early on, 261 was a very good total," said Lara. "Sarwan played really well and gave us a chance.
"But like we did to them in St Lucia (in the fifth and sixth games of the series) they batted well in the first 15 overs. Trescothick played really well and that made it easy for the other guys."
Trescothick hit 12 fours and a six in his 82 to take the game away from West Indies after they had rallied from a slow start to set a challenging total.
Put into bat, Shivnarine Chanderpaul fell for three and Ricardo Powell for nine, both struggling on a lush outfield against mean opening spells from Darren Gough and Steve Harmison.
With Andrew Flintoff also keeping things tight, England restricted West Indies to just 57 from their first 15 overs.
Opener Chris Gayle hit a stylish 41 but when captain Brian Lara fell for eight in the 23rd over, West Indies were struggling on 90 for four.
Lara's dismissal brought Dwayne Smith to the crease and the talented youngster hammered 39 from just 35 balls to lift the run-rate.
SARWAN CENTURY
Sarwan then took control, easing to his second one-day international hundred with 13 fours and adding 68 with Ridley Jacobs (32 not out) for the seventh wicket.
Gough, criticised in the English press after a tired-looking display in Sunday's sixth match of the series, went for 20 in his last two overs but still recorded commendable figures of one for 45 from his 10 overs.
In reply, Trescothick, who made a superb century in the fifth match of the series in St Lucia on Saturday, took the attack to West Indies.
Paceman Mervyn Dillon was the main victim of Trescothick's aggression, the fast bowler giving up 59 from his first eight overs.
Captain Michael Vaughan was adjudged caught behind off Ravi Rampaul for 14, though television replays suggested he did not hit the ball.
With Trescothick firing on all cyclinders, England's 100 came up in the 14th over.
Just when he looked set for a second century in three matches, Trescothick fell to one of the rarer dismissals in cricket, stumped off a wide for 82 from the bowling of Gayle, who was again superb, his eight overs costing just 27.
Scoreboard:
West Indies:
C.Gayle b Anderson 41
S.Chanderpaul c Anderson b Harmison 3
R.Powell run out 9
R.Sarwan not out 104
B.Lara c Read b Clarke 8
D.Smith lbw b Gough 39
D.Bravo b Flintoff 11
R.Jacobs not out 32
Extras (lb-6 w-5 nb-3) 14
Total (for six wickets, 50 overs) 261
Did not bat: I.Bradshaw, R.Rampaul, M.Dillon.
Fall of wickets: 1-16 2-57 3-57 4-90 5-159 6-193
Bowling: Gough 10-1-45-1 (w-2), Harmison 10-0-51-1 (w-2), Flintoff 10-1-45-1 (nb-1), Anderson 8-0-42-1, Clarke 9-0-50-1 (nb-2, w-1), Batty 3-0-22-0
England:
M.Trescothick st Jacobs b Gayle 82
M.Vaughan c Jacobs b Rampaul 14
A.Strauss b Bradshaw 66
R.Clarke c Smith b Bradshaw 9
A.Flintoff c & b Sarwan 11
P.Collingwood not out 46
C.Read not out 23
Extras (lb-5, w-6) 10
Total (for five wickets, 47.2 overs) 262
Did not bat: G.Batty, D.Gough, S.Harmison, J.Anderson.
Fall of wickets: 1-36 2-123 3-146 4-169 5-218
Bowling: Dillon 9-0-67-0 (w-1), Rampaul 4-0-32-1, Powell 6.2-0-35-0 (w-2), Gayle 8-0-28-1 (w-1), Bradshaw 10-0-46-2, Sarwan 9-0-38-1 (w-1), Chanderpaul 1-0-11-0 (w-1)
Result: England win match by five wickets; series drawn 2-2
One-day series results:
First match: Georgetown - England won by two wickets.
Second match: Port-of-Spain - abandoned as a draw due to rain
Third match: Port-of-Spain - abandoned as a draw due to rain
Fourth match: St George's, Grenada - abandoned as a draw due to rain
Fifth match: Castries, St Lucia - West Indies won by five wickets
Sixth match: Castries, St Lucia - West Indies won by four wickets
Seventh match: Bridgetown - England won by five wickets
