It meant the Nottingham 17-year-old finished the event with four medals - one gold, two silvers and a bronze - after being pipped by Russia's Angelina Melnikova by just 0.034 of a point.
Downie's last medal took Great Britain's total haul to six after a previous bronze for James Hall in the men's all-around competition and silver for Courtney Tulloch in the men's rings.
Downie's performances - capped by her historic all-around title on Friday - announced her emergence as a major contender for the Tokyo cycle as well as continuing the recent success of British gymnastics.
Downie told Press Association Sport: "I've always known I'm a good gymnast but I didn't come here expecting to walk out with a gold, two silvers and a bronze so it really blows my mind.
"I think this Championships has really shown me what I'm capable of. I went out and did 12 clean routines and to come away with four medals from them is a great feeling."
Admitting to still feeling a little star-struck after being congratulated by Hungarian pommel star Krisztian Berki, Downie can now expect to be elevated to a new level of recognition herself.
She added: "It's crazy to meet an Olympic champion and I couldn't believe he knew who I was. It's definitely weird to think that other people will be looking out for me now.
"But we do all this hard work so it's great to be recognised. I'm going to get my head down and work on new skills and hopefully show them if I'm selected for the worlds in Montreal."
The event in Canada will see big names like Max Whitlock and Amy Tinkler returning to action but, on the evidence of Romania, Downie looks more than capable of establishing her own headline act.
She admitted to feeling some fatigue when she stepped out for Sunday's remaining two finals but still produced a solid enough performance on the beam to take fourth place.
A single step out in her crowd-pleasing floor routine was probably what cost her a second gold but Downie insisted she could not be downhearted after such a strong series of performances.
She added: "I can't be disappointed about the floor. I knew it was close but the Russian's routine was absolutely flawless so she definitely deserved it."
It was not such a good day for Claudia Fragapane, who fell heavily during her beam routine and also made a mistake on the floor which cost her any chance of a medal.
To the evident delight of the home supporters, Romanian veteran Catalina Ponor held her nerve to take beam gold with her team-mate Larisa Iordache in bronze.
In the final British action of the Championships, Hall once again nailed his exhilarating high bar routine but had to settle for fourth in a high-quality final won by Switzerland's Pablo Bragger.
