Al-Ahsa Oasis, also known as al-Ḥasāʾ (الْحَسَاء) or Hajar (هَجَر), is an oasis and historical region in eastern Saudi Arabia. Al-Ahsa Governorate, which makes up much of the country's Eastern Province, is named after it. The oasis is located about 60 km (37 mi) inland from the coast of the Persian Gulf. Al-Ahsa Oasis comprises four main cities and 22 villages. The cities include Al-Mubarraz and Al-Hofuf, two of the largest cities in Saudi Arabia.

In ecology, an oasis is a fertile area of a desert or semi-desert environment that sustains plant life and provides habitat for animals. Surface water and land may be present, or water may only be accessible from wells or underground channels created by humans. In geography, an oasis may be a current or past rest stop on a transportation route, or less-than-verdant location that nonetheless provides access to underground water through deep wells created and maintained by humans.