Seaward, 29, cut almost four minutes off his personal best as he clocked 2:14.52 - the Rio standard is 2:17.00.
Fellow NI runner Paul Pollock was also inside the standard but his 2:15.38 was only the fourth Irish finisher.
Moldovan-born Sergiu Ciobanu clocked 2:15.14 with Raheny's Mick Clohisey just ahead of Pollock in 2:15.35.
Ireland will only be able to send three male marathon entrants to Rio so Pollock now faces the prospect of having to run quicker next spring if he wants to maintain his Rio dream.
Clonliffe athlete Ciobanu declared for Ireland during the summer and has been a regular on the distance running scene in his adopted country for several years.
"I thought I'd run around 2:15 but I didn't think I could go inside it so I'm absolutely delighted," said Loughborough-based teacher Seaward after his impressive run.
The St Malachy's athlete set his previous personal best of 2:18.46 at the Manchester Marathon last year.
Pollock lamented some recent injury problems which hindered his efforts.
"Three weeks ago I had an 11 centimetre tear in my quad so I wasn't able to run for about 10 days and then it was a week of two or three miles a day," said the 28-year-old, whose time was 52 seconds inside his previous personal best set in Dublin in 2012.
"I caught Mick with four or five miles to go but he caught me with 3K to go.
"In the Brandenburg Gate, it was a kick for the finish and everything seized up and I walked over the line. I'm gutted I'm fourth Irishman."
Lizzie Lee has no such worries over her Rio selection after moving to fifth on the all-time Irish list with a 2:32.51 clocking, which was over nine minutes inside the Olympic standard.
The 35-year-old Cork mother set her previous best of 2:38.09 in Berlin two years ago.
Belfast man Kevin Seaward put himself in line for Olympic Games selection after comfortably achieving the marathon standard  in Berlin.