Dolly Parton secretly traveled to Germany for an experimental cancer treatment about a year before her death ... TMZ has learned.

Sources familiar with the situation tell TMZ ... Dolly privately battled cancer during the final year of her life and sought treatment overseas.

We're told the music icon flew to Frankfurt on a private jet to undergo the treatment. It's unclear exactly what the procedure involved ... but our sources say Dolly remained in Germany for about a week before returning home.

We're told Dolly believed the treatment helped restore her strength and felt it had been effective.

Dolly died August 25 following her private battle with cancer. As we first reported, Dolly's death came as a sudden and unexpected shock to those closest to her ... even though people in her inner circle knew she had been sick.

Our sources previously told us when loved ones learned Dolly had been hospitalized and wasn't doing well, they began rushing to Nashville to be by her side.

Her family held a private ceremony, and she was laid to rest at Woodlawn Memorial Park and Mausoleum in Nashville.

The exact type of cancer Dolly battled, and the nature of the experimental treatment, remain unclear.