Last-gasp Van Poortvliet try edges England to Argentina victory
Replacement Van Poortvliet, 24, crossed with less than two minutes to go as Steve Borthwick's tourists followed up last weekend's win in La Plata with another victory before they finish the season by facing the USA in Washington next Saturday.
Centre Seb Atkinson and full-back Freddie Steward also crossed as fly-half George Ford kicked seven points for the visitors, who claimed their sixth straight Test win -- their best run in five years.
"We found a way in the end," captain Ford told Sky Sports.
"I like our fight, our togetherness, showing how much we care.
"There was some great stuff, we should have scored more tries but the attitude of the group is great," the 32-year-old added.
The Pumas led by three points at the interval thanks to tries from centre Lucio Cinti and winger Ignacio Mendy, but were unable to claim a first home success over England since 2009.
They host Uruguay in Salta next Saturday before opening their Rugby Championship campaign against New Zealand on August 16.
Argentina coach Felipe Contepomi handed flanker Pablo Matera his 111th appearance as the former skipper became the country's most-capped player, taking over from Agustin Creevy.
England were forced into a late change as co-captain Jamie George withdrew from the game hours before kick-off after being called up to the British and Irish Lions squad.
Theo Dan started at hooker instead of George with centre Luke Northmore set for his England debut, replacing the injured Henry Slade.
Northmore started his international career in superb fashion, providing the assist for midfielder partner Atkinson to open the scoring after just four minutes.
Ford, who was England's only captain with George watching from the stands, added the extras to make it 7-0, having set up the try with a delicate cross-kick.
Despite England controlling the first half, Contepomi's side led 17-14 at the break as Cinti and Mendy crossed either side of Steward's effort.
Ford, who played his 100th Test last weekend, brought the sides level 17-17 to set up a tense closing 25 minutes with the visitors on top.
Argentina displayed ill-discipline throughout, and Matera was shown a yellow card with a quarter of the match to play before Van Poortvliet replaced Ben Spencer at half-back.
Despite their dominance, England were finding it difficult to breach Argentina's defence with the first draw in 44 years between the sides looking likely.
With 70 seconds on the clock, England flanker Guy Pepper broke free and fed Van Poortvliet for the dramatic winning try.