Micafungin (MCF) is an antifungal agent of the echinocandin class approved in Europe both in adults and in children for the treatment of invasive candidiasis. Few analytical methods for therapeutic drug monitoring (TDM) of this drug have been described so far. In this paper, we describe a rapid and validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay for the measurement of MCF in plasma. MCF was analyzed in 100-μL plasma samples over a wide range of concentrations (0.1-20 μg/mL) by LC-MS/MS after protein precipitation. The suitability of the assay for TDM was evaluated by using plasma samples from pediatric patients who received MCF for the treatment of invasive candidiasis. The overall turnaround time for the assay was 20 min. The lower limit of quantification of the method was 0.1 ng/mL. No ion suppression due to matrix effects was found with different pre-analytical conditions, such as hemolysis, lipemia, and hyperuricemia. A simple and rapid LC-MS/MS method which provides high specificity, precision, and accuracy for quantification of MCF in plasma has been developed and validated.