Vainio crossed the line in a time of 2:22.26, nearly three minutes quicker than the previous record of 2:25:14, set back in 2014 by Christelle Daunay of France.
The 28-year-old came into the race as the leading contender for gold, having set the European leading time of 2:20.39 in Seville back in February. And she lived up to the favourite tag, finishing nearly five minutes ahead of silver medallist Lili Anna Vindics-Toth of Hungary (2:27:21), before Great Britain's Abbie Donnelly held off another Hungarian, Nora Szabo, to take the bronze.
Finland's Vainio made the victory look like a simple Sunday morning run around the streets of Birmingham, breaking away from the pack with just seven minutes gone, before putting in a faultless display where gold never looked in doubt.
She becomes the first Finnish woman to win a marathon medal at a major senior championships, and the former bandy player takes Finland's second gold of Birmingham 2026, following Silja Kosonen's triumph in the hammer. With Wilma Heltela's silver in the pole vault, Finland sit ninth in the medal table with one evening of action to come.