The Manchester United loanee's ninth-minute strike flew over Thibaut Courtois and into the top corner — the first direct free-kick goal by a Barcelona player in a Clásico since Lionel Messi in October 2012. Ferran Torres added the second in the 18th minute, with Barcelona confirmed as La Liga champions with three games to spare.
Rashford, 28, spoke to BBC Sport after the final whistle and explained how the goal came about.
"No, I wasn't going to shoot because when I put that ball down I didn't see the angles. I didn't feel confident it was going to be a goal, so I was going to cross. But then everyone is telling me to shoot and then I hyped myself up a little bit. It was good I shot in the end as it was a good goal."
He was asked directly about his future and gave the clearest signal yet that he wants to remain.
"I don't know — I'm not a magician. But if I were, I'd stay. I want to win as many things as possible and this is another title that adds to that. This team is great and they will also win a lot in the future. So being part of that would be something special."
Rashford has scored 14 goals and provided 14 assists in 46 appearances this season — making him Barcelona's joint-leading contributor across all competitions. He is the first Englishman to win La Liga in 41 years, since Gary Lineker lifted the trophy with Barcelona in 1985.
His Barcelona future, however, is complicated. The club's €30million purchase option would be triggered by the end of the season and Manchester United are not willing to discuss a second loan or a reduced fee.
United want a permanent departure and are confident of finding a buyer, even if Barcelona do not exercise the option. Barcelona, operating under financial constraints, are hoping to find an alternative arrangement.
The goal at Camp Nou has made that conversation more difficult to ignore — and potentially more difficult for United to control.