The Magpies kicked3.3 to two behinds in the two five-minute periods after full-time to win 13.15(93) to 10.14 (74).
It was the first extra time in a final since the Kangarooseventually beat Hawthorn by 23 in their 1994 qualifying final.
Collingwoodwill now play Geelong next Friday night in their preliminary final.
AlanDidak was best for Collingwood and Anthony Rocca kicked three goals, while
DeanDox and Matt Priddis were best for the Eagles.
The Eagles went into the game missing three of the AFL's biggest names, withcaptain Chris Judd and fellow midfield stars Ben Cousins and Daniel Kerr allinjured.
Leading by three points at half-time, the Eagles kicked the nextthree goals to lead by 23 points more than halfway through the term.
Collingwood looked gone when veteran defender James Clement made his secondclanger of the night, kicking into the man on the mark.
The ball then wentto David Wirrpanda in the goal square, who goaled.
But just like last week,when the Eagles led by 24 points in the third term before Port Adelaide roaredback, the game suddenly turned.
West Coast rested key defender DarrenGlass and Cox late in the term and this opened the door for the Pies.
Travis Cloke goaled and Rocca took full advantage of Glass' absence, kicking twogoals from strong marks.
Instead of being out of the game, the Magpieswent into three-quarter-time only four points behind.
Didak goaled 15minutes into the final term to put Collingwood in front for the first time since20 minutes through the second quarter.
Dale Thomas put Collingwood aheadagain by two points at 23 minutes, before behinds to Glass and Andrew Embleytied the scores.
The Eagles had threatened through the second term,leading by as much as 10 points, but Rocca kicked the last goal of the half tokeep the game tight.
West Coast failed to kick a goal in the opening term,the first time that has happened at Subiaco and only the second time inPerth.
Collingwood suffered a massive pre-game blow when No.1 ruckman JoshFraser was a late withdrawal, apparently with back tightness.
That meantsecond-string ruckmen Guy Richards and Chris Bryan had to go against likelyAll-Australian big man Cox and Mark Seaby.
The Eagles also had to make alate change, bringing in young midfielder Jamie McNamara for defender BeauWaters (hip).
Collingwood looked more dangerous early and gained a boostwhen West Coast key forward Ashley Hansen had to leave the field midway throughthe first term.
Hansen was returning from a fortnight out with a hamstringinjury and did not return.
A poor kick from Cox resulted in the openinggoal of the game, with former Fremantle forward Paul Medhurst taking fulladvantage of the turnover.
The Pies led by seven points at quarter-time,but Cox goaled in the opening minute of the second term.
Leaving aside Adelaide in1999, who failed to make the top eight, West Coast tonight became the firstdefending premier to not win a final the following year since Carlton in1996.Collingwood beat the Eagles in Perth for the firsttime since 1992.
Tonight also continued the extraordinary finals historybetween these two teams, following their draw in 1990 - Collingwood won thereplay under the old system - and a two-point win to the Eagles in 1992.
COLLINGWOOD 1.5 4.8 6.11 10.12 11.13 13.15 (93)
WEST COAST 0.4 5.5 8.9 10.12 10.13 10.14 (74)
After one period of extra time (score after first and second halves of extra time marked in italics).
GOALS
Collingwood: ARocca 3, P Medhurst 2, A Didak 2, D Swan 2, T Cloke, D Thomas, C Bryan, SPendlebury
West Coast: D Wirrpanda 2, M Le Cras 2, D Cox,A Embley, C Fletcher, Q Lynch, M Priddis, M Rosa
BEST
Collingwood: ADidak, P Medhurst, S Pendlebury, D Swan, T Goldsack
West Coast: D Cox, M Priddis, B Jones, D Chick,D Glass
INJURIES
Collingwood: J Fraser (back)replaced in selected side by C Byrne, S Burns (ankle)
West Coast: B Waters(hip) replaced in selected side by J McNamara, A Hansen (hamstring)
Reports: None
Umpires: S McBurney, S Ryan, S McInerney.
Crowd: 43,627 at Subiaco Oval, Perth