The Love Album beat the three other new releases to number one spot and the Irish boyband's Nicky Byrne said they were "absolutely thrilled".
But the week's real surprise package was Wales's Fron Male Voice Choir, whose members have an average age of 66, which reached number 13.
Oasis made it to number two with their "best of" package Stop The Clocks and the Fab Four were third with their re-mix album Love.
U218 Singles - U2's greatest hits album - came in at number four, while George Michael's Twenty Five, also a hits compilation, dropped from top spot to five.
Byrne said: "We are absolutely thrilled to be Number One. It means so much to us. We have the best fans in the world and we want to thank them for all their continued support."
The band's manager Louis Walsh said: "It just shows there is true, genuine demand for Westlife when they can beat such amazing legends as the Beatles, U2 and Oasis.
"Westlife are up there with the best. This is their eighth album and proves there is still a long way to go for them.
Dedicated
"I can't tell you how proud I am of the boys. They have got the best and most dedicated fans in the world."
HMV's Gennaro Castaldo said: "I guess it shouldn't come as that much of a surprise when you consider their amazing run of chart success in recent years."
He said their "devoted fanbase" had swelled over time and an appearance on the X Factor last week helped boost sales and gave them a crucial advantage.
"Westlife's narrow chart success is no discredit to the other bands, however, who ran them very close in the end," Mr Castaldo added.
Urgency
"'Best Of' albums tend to sell more copies over a longer period, as fans usually show less urgency in buying them.
"So while Oasis and The Beatles may have just missed out topping the charts this week, they're well placed to be among the biggest sellers this Christmas."
Meanwhile, the high placing for the Fron Male Voice Choir, which was the next highest new entry after U2, was the latest boost for the north Wales pensioners.
With its 66 members having a combined age of 3,979, their album Voices of the Valley went gold within three days of going on sale and is the first time a male voice choir has achieved such chart success.
Plans are now afoot for a series of television appearances and a Hollywood film charting their 60-year "rollercoaster tale" from a tiny village choir to stardom.
The choir comes from Froncysyllte in the vale of Llangollen, near Wrexham.
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