Harold Bluetooth Gormson (Danish Harald Blåtand, Norwegian Harald Blåtann, German Harald Blauzahn) (c. 935 – November 1 986), sometimes Harold II, succeeded his father Gorm the Old as king of Denmark in 958 (or 959) and was king of Norway for a few years, probably around 970.

Although his predecessors had accepted Christianity at the instigation of the Frankish Carolingian kings in 826, many Danes and other northerners were still heathens for centuries. Harald Bluetooth embraced Christianity around 960 or 965, but whether this was a purely political move or the result of an actual conversion on Harold's part is impossible to say. His son Sweyn (Forkbeard) was baptized along with the rest of the royal family, and given the name of the Holy Roman emperor Otto the Great. Harold was wounded in battle against the forces of his son and successor Sweyn. He is believed to have died on November 1st, 986, although 985 and 987 are also mentioned, and is best remembered for having erected a large runic stone at Jelling:

"Harold, king, bade these memorials to be made after Gorm, his father, and Thyra, his mother. The Harald who won the whole of Denmark and Norway and turned the Danes to Christianity."

(In Old Norse: Haraltr kunukr bath kaurua kubl thausi aft kurm fathur sin auk aft thaurui muthur sina sa haraltr ias sar uan tanmaurk ala auk nuruiak auk tani karthi kristna.)

Norway was ruled by his vassal Håkon Jarl, who had joined with Harold Bluetooth in a conspiracy against Harald Greyhide, and who now in reality controlled the country. Håkon Jarl broke his alliance when king Bluetooth attempted to force Christianity upon him. Having resisted subsequent attack from Denmark, Håkon became de facto sole ruler of ruler of Norway to 995.

Harold may have had three wives or consorts: Thora, Gunhilde and Gyrid (the niece of the Swedish king Eric the Victorious). He is believed to have had four children: Håkon, Sweyn I, Gunhild and Tyra (who married Styrbjörn Starke).

The Bluetooth interface for wireless Personal Area Networks developed by Ericsson is named after Harold. The Bluetooth logo consists of the Nordic runes for his initials, H and B.