Brazilian striker Hulk cancelled out Maurinho's opener for the South Korean side with a powerful long-range strike before goals from Zheng Wei and Wu Lei gave SIPG a two-goal cushion at halftime.
Park Chu-young reduced the deficit for FC Seoul before Oscar scored SIPG's fourth as Andre Villas-Boas' team guaranteed themselves a top-two finish in Group F alongside Urawa.
"Even though we conceded at the beginning, we stayed composed and managed to come back," said the Portuguese coach.
"Individually and as a team we were very good. Every player gave everything they had, and we nailed down a qualifying spot from a difficult group."
Urawa comfortably dismissed 2014 champions Western Sydney Wanderers, with the Japanese side notching up a 6-1 win in which Rafael Silva scored twice as the J.League club also earned a berth in the last 16.
Mihailo Petrovic's team are joined in the next round by Thailand's Muangthong United, who defeated Brisbane Roar in Group E thanks to goals from former Newcastle United striker Xisco Jimenez, Chanathip Songkrasin and Teerasil Dangda while Kashima Antlers moved a step closer to advancing with a 4-0win over former winners Ulsan Hyundai.
Also progressing to the knockout rounds are last year's beaten finalists Al Ain from the United Arab Emirates, who defeated Iran's Zobahan to ensure they cannot finish outside the top two in Group C. Bunyodkor's surprise 2-0 win over Al Ahli, meanwhile, stalled the Saudi Arabian club's hopesof progress from the same group for another fortnight.
A 3-1 win over Al Jazira of the UAE secured Qatar's Lekhwiya a spot in the next round, with Esteghlal Khouzestan and Al Fateh from Saudi Arabia fighting for second place in Group B after their 1-1 draw.
Two-time champions Al Hilal of Saudi Arabia are also through following their 0-0 draw with Persepolis, and the Iranians remain in the hunt for Group D's second berth in the last 16 after Al Wahda thrashed Qatar's Al Rayyan 5-1.
Luiz Felipe Scolari's Guangzhou Evergrande picked up only their second win of the group stages on Tuesday evening as the Chinese Super League champions demolished Hong Kong's Eastern 6-0, with former Tottenham Hotspur midfielder Paulinho scoring twice.
Guangzhou lead Group G by a point from Suwon Bluewings of South Korea, who were defeated 1-0 by Kawasaki Frontale as the Japanese side won their first game of the campaign.
In Group A, three points covers the four teams with one match remaining after Al Ahli of the UAE were held to a 1-1 draw by Iran's Esteghlal and Saudi Arabia's Al Taawoun and Lokomotiv Tashkent played out an extraordinary 4-4 draw.
(Reporting by Michael Church in Hong Kong; Editing by Hugh Lawson)
