Importance: Medical Device Safety Communications (MDSCs) are used by the US Food and Drug Administration (FDA) to convey important new safety information to patients and health care professionals. The sources of initial safety signals that trigger MDSCs have not been described previously. Objective: To assess the sources of initial safety signals that trigger publication of MDSCs and the potential associations among MDSC data source, type of safety issue, and subsequent FDA action. Design, Setting, and Participants: In this cross-sectional study, all MDSCs published on the FDA website between January 1, 2011, and December 31, 2019, were assessed. The MDSC characteristics, sources of initiating safety signals, regulatory approval or clearance pathways of the related medical devices, and subsequent FDA actions were collected from the FDA website. Main Outcomes and Measures: The main outcome was the distribution of sources of initial safety signals that led to publication of MDSCs. Secondary aims included exploration of potential associations among safety signal sources (direct reporting vs other), type of safety issue (death vs other), and FDA action (withdrawal vs other). Results: A total of 93 MDSCs were evaluated. Median time from device approval to MDSC posting was 10 years (interquartile range, 6-16 years). The most common data sources that triggered MDSCs were direct reports to the FDA through the Medical Device Reporting (MDR) program (44 of 93 [47%]) followed by regulator-initiated assessments (32 [34%]). Common safety issues included patient injury (25 [27%]), potential wrong diagnoses (19 [20%]), and death (18 [19%]). Frequent FDA action after MDSC posting included recommendation for increased vigilance and caution (47 [51%]), complete device withdrawal (12 [13%]), and warnings of specific lots or clinics (12 [13%]). There was a statistically significant correlation between direct reports of adverse events to the FDA through the MDR program and risk of death as a safety issue (14 of 44 [32%] for direct reporting vs 4 of 49 [8%] for any other data sources, P =.007). Conclusions and Relevance: In this cross-sectional study, the most common source of initial safety signals that triggered MDSCs was direct reports of real-world adverse events to the FDA through the MDR program. The delayed detection of postmarketing adverse events highlights the importance of proactive identification of emerging device-related safety issues..