Introduction Hospital readmissions after elective shoulder arthroplasty represent a costly burden to the health-care system due to the rising demand for this procedure. Nevertheless, the reasons for readmissions remain inconclusive. Therefore, the main purposes of the current investigation were to determine (1) the readmission rates within 90 days after shoulder arthroplasty, (2) the reasons behind those readmissions, and (3) the characteristics of readmitted patients. Methods A retrospective review was performed on 184 patients who underwent total shoulder arthroplasty (TSA) or reverse shoulder arthroplasty (RSA) performed by a single surgeon at a single institution between 2010 and 2016. The readmissions that occurred within 90 days after surgery and their reasons were noted. Patient characteristics were compared between readmitted and non-readmitted patients. Results Overall, 7.1% of patients were readmitted within 90 days after shoulder arthroplasty. However, patients with a previous surgery on the same shoulder of the TSA or RSA had a higher rate (13.5%) than patients without a previous surgery (5.4%). Readmission rates for TSA and RSA were virtually identical. Most readmissions were due to surgical (69.2%) rather than medical complications (30.8%). The most common reasons for readmission were surgical-site infection and deep vein thrombosis. There were no statistically significant differences between the baseline patient characteristics of readmitted and non-readmitted patients. However, the proportion of males (69.2%) was more than twice the one of females (30.8%) among readmitted patients. Conclusion The readmission rate within 90 days after surgery was relatively low among patients who underwent either a TSA or RSA at our institution. Surgically related causes were largely responsible for readmissions.