There was a decline in sales and income caused by customer behavior in purchasing decisions. To improve customer behavior in purchasing decisions, research was conducted regarding service quality, promotions, and product availability which can influence purchasing decisions at Watsons Medan. Data collection was carried out using questionnaire techniques and literature review techniques. Data were processed using validity, reliability, multiple linear regression, determination tests, t-tests, and F-tests, complemented by classical assumption tests. Measurements were carried out by distributing questionnaires to 30 respondents and then testing their validity and reliability. After that, it was distributed to 120 respondents for analysis. The results of this research found that service quality, product availability, and promotion simultaneously have a significant influence on purchasing decisions. However, partially service quality and product availability have less influence on purchasing decisions at Watsons Medan; only promotions have a significant influence on purchasing decisions at Watsons Medan. / Terjadi penurunan di penjualan dan pendapatan yang disebabkan oleh perilaku pelanggan dalam keputusan pembelian. Untuk meningkatkan perilaku pelanggan dalam keputusan pembelian maka dilakukan penelitian mengenai kualitas pelayanan, promosi, dan ketersediaan produk yang dapat mempengaruhi keputusan pembelian di Watsons Medan. Pengumpulan data dilakukan dengan teknik kuesioner dan teknik kepustakaan. Data diolah dengan menggunakan uji validitas, reliabilitas, regresi linear berganda, uji determinasi, uji-t, dan uji-F, dilengkapi dengan uji asumsi klasik. Pengukuran dilakukan dengan menyebarkan kuesioner kepada 30 orang responden, kemudian diuji validitas dan reliabilitasnya. Setelah itu dilakukan penyebaran kepada 120 orang responden untuk dianalisis. Hasil penelitian ini didapati bahwa kualitas pelayanan, ketersediaan produk dan promosi secara stimultan berpengaruh signifikan terhadap keputusan pembelian. Namun, secara parsial kualitas pelayanan dan ketersediaan produk kurang berpengaruh terhadap keputusan pembelian di Watsons Medan; hanya promosi yang memberikan pengaruh signifikan terhadap keputusan pembelian di Watsons Medan.