Hutchinson-Gilford progeria syndrome (HGPS) is a premature aging disease caused by a truncated lamin A protein (progerin) that drives cellular and organismal decline. HGPS patient-derived fibroblasts accumulate genomic instability, but its underlying mechanisms and contribution to disease remain poorly understood. Here, we show that progerin-induced replication stress (RS) drives genomic instability by eliciting replication fork (RF) stalling and nuclease-mediated degradation. Rampant RS is accompanied by upregulation of the cGAS/STING cytosolic DNA sensing pathway and activation of a robust STAT1-regulated interferon (IFN)-like response. Reducing RS and the IFN-like response, especially with calcitriol, improves the fitness of progeria cells and increases the efficiency of cellular reprogramming. Importantly, other compounds that improve HGPS phenotypes reduce RS and the IFN-like response. Our study reveals mechanisms underlying progerin toxicity, including RS-induced genomic instability and activation of IFN-like responses, and their relevance for cellular decline in HGPS. Display omitted •Replication stress in progerin-expressing cells is caused by fork stalling and degradation•Progerin activates cGAS/STING pathway and a robust STAT1-regulated IFN-like response•Replication stress and IFN-like response contribute to HGPS cellular aging phenotypes•Calcitriol reduces replication stress and IFN-like response, rejuvenating HGPS cells Kreienkamp et al. reveal mechanisms underlying cellular decline in the premature aging disease Hutchinson-Gilford progeria syndrome. Progerin, the mutant protein that causes this disease, elicits replication stress and a cell-intrinsic innate immune response. The study identifies strategies, such as calcitriol, that rescue these phenotypes and rejuvenate progeria cells.