The 15-year-old was driving a red Honda quad bike on the A711 just north of Mutehill Village near Kirkcudbright when the crash happened at about 1.20 pm on Saturday.
Emergency services attended and the boy was airlifted to the Queen Elizabeth University Hospital in Glasgow by helimed.
The road was closed for six hours whilst investigations were carried out.
Inspector Greg Dinnie said: “A teenager has sustained serious injuries and it is important that we establish how this crash occurred.
“We are appealing for anyone who may have been driving in this area prior to the crash and seen a red coloured quad bike to come forward.
“You may also have been in the area and have dash-cam footage that can help us, please check back and get in touch.”
Anyone with any information should contact police via 101, quoting incident number 1739 of Saturday July 2.
