甲型肝炎在中国大陆常简称甲肝。是一种发生于肝脏的急性感染疾病，起因于A型肝炎病毒（hepatitis A virus, HAV）。在许多案例当中，患者只有少许症状或无症状，尤其是年纪较轻的病患。从感染到发病，其潜伏期约为二至六周。如果出现症状，持续时间通常为八周，症状可能包含：恶心、呕吐、腹泻、皮肤发黄、发烧和腹痛。约10至15%的病患，在初次感染后6个月内会出现复发症状。严重病患可能出现急性肝功能衰竭，这在老年患者中比较常见。

A型肝炎最常见的传染途径，是食入或饮用到被感染者粪便污染的食物或饮水，有时也可见到因为食用未煮熟的贝类而感染的病例，与患者有密切接触也可能会因此感染。儿童感染A型肝炎时经常没有症状，但他们仍然具有传染力。在感染过A型肝炎后，患者终身对A型肝炎免疫。由于A型肝炎的状况与许多疾病类似，需要血液检验才能确诊。

Hepatitis A (formerly known as infectious hepatitis) is an acute infectious disease of the liver caused by the hepatitis A virus (HAV). Many cases have few or no symptoms, especially in the young. The time between infection and symptoms, in those who develop them, is between two and six weeks. When symptoms occur, they typically last eight weeks and may include nausea, vomiting, diarrhea, jaundice, fever, and abdominal pain. Around 10–15% of people experience a recurrence of symptoms during the six months after the initial infection. Acute liver failure may rarely occur with this being more common in the elderly.