莫西沙星（学名：Moxifloxacin，发音「moxi FLOX a sin」）是一种由拜尔公司开发的第四代人工合成的喹诺酮类抗细菌药（8-methoxy-fluoroquinolone类抗生素）。其口服剂（以盐酸莫西沙星之形式）在世界范围内以Avelox、Avalox和 Avelon等商标名贩售，在大多数国家也有静脉注射剂。该药也有治疗结膜炎（红眼病）的眼药水，商标名为Vi****x和Moxeza。其杀菌作用借由干扰拓扑异构酶（Topoisomerase），进而抑制DNA合成，最终导致细菌死亡，1999年12月由FDA核准上市。

Moxifloxacin is a fourth-generation synthetic fluoroquinolone antibacterial agent developed by Bayer AG (initially called BAY 12-8039). It is marketed worldwide (as the hydrochloride) under the brand names Avelox, Avalox, and Avelon for oral treatment. In most countries, the drug is also available in parenteral form for intravenous infusion. Moxifloxacin is also sold in an ophthalmic solution (eye drops) under the brand names Vi****x, and Moxeza for the treatment of conjunctivitis (pink eye). Its antibacterial spectrum includes enteric Gram-(−) rods (Escherichia coli, Proteus species, Klebsiella species), Haemophilus influenzae, atypical bacteria (Mycoplasma, Chlamydia, Legionella), and Streptococcus pneumoniae, and anaerobic bacteria. It differs from earlier antibacterials of the fluoroquinolone class such as levofloxacin and ciprofloxacin in having greater activity against Gram-positive bacteria and anaerobes. Because of its potent activity against the common respiratory pathogen Streptococcus pneumoniae, it is considered a "respiratory quinolone."