PBS’ “Downton Abbey” drew 5.4 million viewers for its second-season finale Sunday, the pubcaster’s best performance since the premiere of Ken Burns’ “National Parks” in September 2009.

Airing under the umbrella of PBS’ “Masterpiece,” “Downton” improved 25% on average from its first season to its second and doubled the PBS primetime average. Its overall audience Sunday was on par with ABC’s combo of “Desperate Housewives” and the season finale of “Pan Am,” which averaged 5.7 million viewers from 9-11 p.m.

Compared with the “Masterpiece” season average from 2010-11, “Downton” had generated a 251% increase among women 18-34, 145% among women 35-49, 111% among men 18-34, 84% among men 35-49 and 88% among teens 12-17.

” ‘Downton Abbey’ has become a cultural phenomenon,” executive producer Rebecca Eaton said. “It is so gratifying to see our beloved ‘Masterpiece,’ after more than 40 years on PBS, attracting a whole new audience.”

“Downton” has attracted an additional 1.5 million unique visitors to streams of the period drama on PBS’ website.

“From viewing parties to Twitter mentions, ‘Downton Abbey’ is riding a wave of public enthusiasm,” PBS prexy and CEO Paula Kerger said, “and it’s been wonderful to see so many people discovering public television as a destination for programming that’s smart, distinctive and entertaining.”

“Downton Abbey” is a Carnival/Masterpiece co-production.