化学疗法（英语：Chemotherapy），简称化疗（Chemo），是用特殊的药物来治疗疾病，为目前治疗肿瘤及某些自身免疫性疾病的主要手段之一，不过在治疗中，普遍会为患者带来明显的恶心及呕吐等副作用，为患者带来不适感。化疗是指应用药物治疗癌症。这些可杀灭肿瘤细胞，有时称为细胞毒药物。许多化疗药物来源于自然，如：植物，其他是人工合成。目前已超过50种化疗药物，常用的有：表阿霉素、阿霉素、柔红霉素、丝裂霉素、氟脲嘧啶脱氧核苷酸等。这些药物经常以不同的强度联合应用。

化疗是一种全身性治疗手段对原发灶、转移灶和亚临床转移灶均有治疗作用，但是化疗治疗肿瘤在杀伤肿瘤细胞的同时，也可能将正常细胞和免疫（抵抗）细胞一同杀灭，所以化疗是一种"两害相权取其轻"的治疗手段。

Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a category of cancer treatment that uses chemical substances, especially one or more anti-cancer drugs (chemotherapeutic agents) that are given as part of a standardized chemotherapy regimen. Chemotherapy may be given with a curative intent (which almost always involves combinations of drugs), or it may aim to prolong life or to reduce symptoms (palliative chemotherapy). Chemotherapy is one of the major categories of medical oncology (the medical discipline specifically devoted to pharmacotherapy for cancer).