Cardiac rhabdomyomas are the most common pediatric cardiac tumors and are frequently associated with tuberous sclerosis complex (TSC). Large fetal cardiac rhabdomyomas causing inflow or outflow obstruction can lead to hemodynamic compromise and arrhythmias, sometimes necessitating emergent perinatal interventions. We present the fetus of a 32-year-old with multiple intracardiac rhabdomyomas identified at 23 weeks gestation. The largest, located in the left atrium (LA), enlarged by 32 weeks, causing mitral inflow obstruction and moderate mitral regurgitation (MR). Maternal sirolimus therapy was initiated, leading to tumor regression and permitting delivery at 39 weeks without the need for fetal or neonatal surgery. Postnatally, the infant developed hemodynamically significant refractory arrhythmias, requiring antiarrhythmics and inotropic support. Continued sirolimus therapy further reduced tumor burden and improved MR. This case adds to the growing evidence supporting transplacental sirolimus therapy (mTOR inhibition) for managing large, hemodynamically significant fetal cardiac rhabdomyomas. It also underscores the importance of anticipating and aggressively managing associated arrhythmias.