D66 may refer to:

• Queen's Gambit Declined, Encyclopedia of Chess Openings code
• D66 (airport), an American public airport
• d66 (die), a dice roll used in some role-playing games and old wargames
• D66 road (Croatia), a state road in Croatia
• Democrats 66, commonly known as D66, a political party in the Netherlands
• HMS Emerald (D66), an Emerald-class light cruiser
• D66 Strain of Chlamydomonas reinhardtii

• Small nucleolar RNA SNORD98
• Small nucleolar RNA SNORD99
• Small nucleolar RNA snoZ7/snoR77
• Small nucleolar RNA snR48
• Small nucleolar RNA snR53
• Small nucleolar RNA snR54
• Small nucleolar RNA snR55/Z10
• Small nucleolar RNA snR57
• Small nucleolar RNA snR60/Z15/Z230/Z193/J17
• Small nucleolar RNA snR61/Z1/Z11
• Small nucleolar RNA snR64
• Small nucleolar RNA snR66
• Small nucleolar RNA snR69
• Small nucleolar RNA snR71
• Small nucleolar RNA TBR17
• Small nucleolar RNA TBR5
• Small nucleolar RNA TBR7
• Small nucleolar RNA U2-19
• Small nucleolar RNA U2-30
• Small nucleolar RNA U3
• Small nucleolar RNA U54
• Small nucleolar RNA U6-53/MBII-28
• Small nucleolar RNA U83B
• Small nucleolar RNA Z101
• Small nucleolar RNA Z102/R77