Intermediate‐volatility organic compounds (IVOCs) are among the most important precursors to secondary organic aerosol (SOA), yet their sources and contributions to SOA in ambient air are poorly constrained. In this study, IVOCs were collected with sorption tubes in a coastal city in southern China during September–October 2019 and were analyzed by gas chromatography‐mass selective detector after thermo‐desorption. The measured average concentration of IVOCs was 25.0 ± 0.95 μg m−3 (mean ± 95% C.I.), and residual unresolved complex mixtures shared 79.8% ± 1.91% of IVOCs. The estimated SOA production, even only from speciated IVOCs and unspeciated branch‐alkane IVOCs, reached 2.44 ± 1.46 μg m−3, approximately five times that from VOCs during the photochemically active period (12:00–15:00 local time). Based on the positive matrix factorization model with a photochemical‐age‐based parameterization, diesel‐related emission was the largest contributor (46.6%) of IVOCs, followed by ship emission (23.0%), gasoline exhaust (16.8%), and biomass/coal burning (13.6%). Non‐road diesel engines accounted for a dominant part in diesel‐related emission. Ship emission was found to contribute SOA formation potentials (SOAFPs) comparable to that of diesel‐related emission, while biomass/coal burning showed higher SOAFPs than gasoline exhaust. Our results revealed that non‐road sources, such as ship emission, non‐road diesel engines, and biomass/coal burning contributed substantially to IVOCs, and will be of greater importance in producing ambient SOA with the increasingly stringent control on emissions from on‐road vehicles.