The two-hour finale, which also featured performances from Take That, averaged a colossal 16.08m (51.5%) for ITV1 from 7.30pm, with a further 1.08m (3.5%) watching on ITV1 HD. Overnight ratings figures do not take into account viewers who may have been watching in pubs or clubs.
The figure is on par with that for the finale of the 2009 series, when a peak of 19.7m (63.2%) watched as Joe McElderry was named the winner.
Last night's peak came at 9.20pm, when 18.14m (56.4%) were tuned to ITV1 and 1.3m (4.1%) to ITV1 HD.
Spinoff show The Xtra Factor commanded a similarly huge audience, drawing an average of 2.31m (9.6%) for ITV2 between 9.30pm and 10.45pm. A further 175k (1.1%) watched on ITV2+1.
On BBC One, the latest Strictly Come Dancing results show - in which two couples were eliminated - drew an average of 11.58m (41.1%) between 7pm and 7.30pm.
Later on the channel, Antiques Roadshow mustered up 5.71m (18.4%) from 7.30pm, then Inspector George Gently took 4.15m (13.8%).
After The X Factor on ITV1, 5.24m (22%) remained to see Piers Meets Elton: A Life Stories Special.
BBC Two held a respectable audience opposite the commercial competition, averaging 1.94m (7.8%) with its snooker coverage between 8pm and midnight.
Channel 5's movie The One had 681k (2.2%) from 7.50pm, then the audience rose to a commendable 1.92m (9.2%) for Dirty Dancing at 9.30pm.
On Channel 4, Celebrity Come Dine With Me managed 900k (2.8%), then Any Human Heart delivered 990k (3.5%).
Overall in primetime, ITV1 was on top with an average share of 37.8% and 2.3% for ITV1 HD. BBC One had 18.8% for second, followed by BBC Two with 6.4%, Channel 4 with 4% (+1: 0.7%) and Five with 4.3%.
BARB ratings data supplied by Attentional