Monolayer lateral heterostructures (LHSs) composed of two-dimensional semiconducting ferroelectrics hold promise for non-volatile electronic and optoelectronic applications. Lattice misfit strain at these atomically sharp interfaces can significantly modulate in-plane polarization and related electronic properties, yet the direct characterization of the strain profile remains challenging. Here, we map the strain in SnTe-PbTe monolayer LHSs quantitatively via low-temperature scanning tunneling microscopy (STM). By combining moiré pattern simulation and geometric phase analysis, we achieve lattice parameter measurements with 0.02 Å precision—which substantially exceeds conventional STM resolution. Significant tensile strain is observed in SnTe regions, while PbTe sections exhibit much smaller strain. Density functional theory calculations attribute this asymmetric strain distribution to distinct atomic coordination in each monolayer semiconductor. These findings offer critical insights into strain engineering of ultrathin group-IV monochalcogenide heterostructures.