Identifying Galactic PeVatrons (PeV particle accelerators) from ultrahigh-energy (UHE, >100 TeV) γ-ray sources plays a crucial role in revealing the origin of Galactic cosmic rays. The UHE source 1LHAASO J1857+0203u is suggested to be associated with HESS J1858+020, which may be attributed to the possible PeVatron candidate supernova remnant (SNR) G35.6?0.4 or H ii region G35.6?0.5. We perform detailed analysis on the very-high-energy and UHE γ-ray emissions toward this region with data from the Large High Altitude Air Shower Observatory (LHAASO). 1LHAASO J1857+0203u is detected with a significance of 11.6σ above 100 TeV, indicating the presence of a PeVatron. It has an extent of ～0 . ° 18 with a power-law (PL) spectral index of ～2.5 at 1–25 TeV and pointlike emission with a PL spectral index of ～3.2 above 25 TeV. Using archival CO and H i data, we identify some molecular and atomic clouds that may be associated with the TeV γ-ray emissions. Our modeling indicates that the TeV γ-ray emissions are unlikely to arise from clouds illuminated by the protons that escaped from SNR G35.6?0.4. In the scenario in which H ii region G35.6?0.5 could accelerate particles to the UHE band, the observed GeV–TeV γ-ray emission could be well explained by a hadronic model with a PL spectral index of ～2.0 and cutoff energy of ～450 TeV. However, an origin in an evolved pulsar wind nebula cannot be ruled out.