An ostium (pl.: ostia) in anatomy is a small opening or orifice.

Ostia, not as a plural, is also the name of a number of places.

Ostium or ostia may refer to:

• Ostium of fallopian tube
• Ostium of the uterus (disambiguation)
• Ostium primum of the developing heart
• Ostium secundum (foramen ovale) of the developing heart
• Ostium maxillare of the maxillary sinus
• Ostium vaginae (vaginal orifice)
• Coronary ostium – opening of coronary arteries at root of aorta, superior to aortic valve
• Sinus ostium an opening that connects a sinus to the nasal cavity itself.

• Ostium (sponges), the pores of living sponges
• Ostium (insect anatomy), of female Lepidoptera genitalia

• All pages with titles containing Ostium
• Pore (disambiguation)
• Foramen
• Ostiole