Lena Jönsson found the balloon on Monday during a mushroom hunt in Östersund.
Attached was a message in English.
"It was written by a woman who had lost her husband to cancer, he was just 40 years old," Jönsson told the Aftonbladet newspaper.
She said that she understood that multiple balloons had been released at the husband's funeral.
The message ended with an address in Wigan, which is near Manchester in northern England, and a request to mail the letter back.
There was no name or date on the message, but the deceased man's wife had written that she wanted all the messages back to help her grieve.
"I don't know when this was sent or how long the balloon was sitting in the forest, but the whole thing is fantastic," Jönsson told the paper.
"I'm going to write a letter. It would be so lovely to know more."
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