Two Gustav Mogensen goals put the young Bees in the ascendancy before their opponents pulled one back just before half-time through Yang Yumin.
Matej Majka then added a third when he finished well in the second half but goals from Han Chanhee and Park Dongjin ensured the game would end level.
Neil MacFarlane’s side started the brighter with the energy and press from the front three causing the South Korean side multiple problems.
The first chance came after a couple of minutes when hard work from Mogensen saw him charge down the goalkeeper’s clearance, but the ball fell kindly for Seoul as they managed to clear.
Just 10 minutes later, though, The Bees would have their lead in the same circumstances as the first chance as Mogensen managed to slide and block the clearance into the Seoul net.
That preceded a spell where Seoul managed to get a foothold in the game as chances fell for Kim Wonsik and Lee Sungjae.
Despite them seeing more of the ball, The Bees carried an excellent threat on the counterattack, and it paid off on 35 minutes when Mogensen received the ball well before playing a one-two with Yearwood and driving at goal before slotting calmly into the net.
It was almost three in a similar move just a couple of moments later when Yearwood won the ball back high before driving into the box and cutting the ball back towards Mogensen, but he was unable to connect.
Yumin then made sure that his side would get on the scoresheet before the break after good individual skill saw him drive in from the left before lashing an effort high into the goal past Nathan Shepperd.
The Bees then started the second half similar to the first and just seconds in a ball over the top would find Mogensen who latched onto it well before shooting, but the onrushing keeper did well to make the block.
The third did come just a few minutes later when Yearwood found Mogensen whose first touch took him away from the Seoul defenders before he threaded the ball through to Majka on the right who shot low across goal to make it 3-1.
FC Seoul then managed to gain more of the ball going into the latter stages of the second period and they got their second on 74 minutes when they did well down the right before a low cross was sent into the box before Chanhee turned the ball home with a first time finish.
The third followed shortly after Alibaev hit a powerful free-kick which smashed the crossbar and bounced down before Dongjin managed to head the ball home after the ball bounced down off the line.
A late chance then fell to substitute Christian Tue Jensen with a free-kick but he could only direct his effort straight at the Seoul wall as the match finished 3-3.
Brentford B: Shepperd, Larsen, Tsaroulla, Yearwood (Maghoma, 61), Racic, Oksanen (Read, 59) Majka, Hammar (Carre, 78) Trialist (Tue Jensen, 59), Mogensen, Titov (O’Connor, 78)
Subs: Trialist, Koutsimouka, Adams, Mitchell
FC Seoul: Yang Hanbeen, Hwang Hyunsoo, Osmar, Ju Sejong, Alibaev, Park Chuyoung, Kim, Wonsik, Kim Jusung, Lee Sungjae, Ko Kwangmin, Yang Yumin
Subs: Yu Sanghun, Lee Wonghee, Kim Namchun, Kang Sanhee, Kim Hangil, Shin Jaewon, Cha Oyeon, Jung Wonjin, Kim Minsoo