The Great Britain pair were leading going into the final round at the London Aquatic Centre and only need a score of 51 points to win.
But teenager Toulson landed flat on the water and they only scored 38.40 for their last effort, which was only good enough for fifth.
The duo had been favourites for the title after finishing third overall in the recent World Series behind China and Canada.
Germany’s Maria Kurjo and My Phan won with 279.75 points, with Ukraine’s Ganna Krasnoshlyk and Vlada Tatsenko were second on 278.01. Hungary’s Gyongynver Kormos and Zsofia Reisigner claimed a surprise third with 274.74, with Couch and Toulson scoring 267.72.
Couch, who had won the European synchro title with Sarah Barrow in 2012, said afterwards: “We’re a little bit gutted tonight but she’s (Toulson) such an amazing diver and we’ve got so much more to give. We’re going to let that one go and just prepare for the summer ahead.”