Ha may refer to:

• Hahnium, an element now called Dubnium
• Hartree, an atomic unit of energy
• Hectare (ha), a unit of area
• Hectoampere, a unit of electric current
• Hour angle, in astronomy, one of the coordinates of the equatorial coordinate system
• Hyaluronan (Hyaluronic acid), a carbohydrate structure
• Hydroxylapatite, a mineral
• High availability, systems design and implementation with a view to maximising service

Hà is a Vietnamese surname. The name is transliterated as He in Chinese and Ha in Korean.

Hå is a municipality in Rogaland county, Norway. Hå is the southernmost municipality in the district Jæren.

H-alpha (Ha) is a specific deep-red visible spectral line in the Balmer series created by hydrogen with a wavelength of 656.28 nm, which occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha light is important to astronomers as it is emitted by many emission nebulae and can be used to observe features in the sun's atmosphere including solar prominences.

or (also spelled Khet, Kheth, Chet, Cheth, Het, or Heth) is the eighth letter of the Semitic abjads, including Phoenician Ḥēt , Hebrew Ḥēt , Aramaic Ḥēth , Syriac Ḥēṯ , and Arabic Ḥā' .