Respiratory sinus arrhythmia (RSA), an index of the parasympathetic nervous system, has recently gained attention as a physiological component of regulatory processes, social connectedness, and health. Within the context of romantic relationships, studies have operationalized and conceptualized RSA in disparate ways, obscuring a clear pattern of findings. This systematic review synthesizes the rapidly developing literature and clarifies the role of RSA in romantic relationships. We evaluate support for three conceptual hypotheses: (1) resting baseline RSA is associated with better quality relationships; (2) phasic RSA is reflective of changes in threat and connection during couple interactions; and (3) physiological linkage in RSA between romantic partners relates to positive or negative relationship functioning depending on the nature of the linkage (e.g., in‐phase vs. antiphase). We identified 26 empirical studies that tested associations between RSA and an index of romantic relationships (i.e., relationship satisfaction). Our findings show that higher RSA is not uniformly “good” for relationships. Higher resting baseline RSA was contemporaneously associated with better quality relationships, yet higher baseline RSA was also unexpectedly associated with relationship violence. Short‐term decreases in RSA were found during relationship conflict, though the opposite—phasic increases in RSA during positive romantic partner interactions—was not found due to mixed empirical support. As expected, evidence for RSA linkage was found, though the connection between linkage and relationship functioning depends on the context in which it was measured. We discuss methodological limitations and directions for future research.