Secant piles are effective as deep excavation-retaining structures, providing structural support and stability, preventing soil collapse, and reducing groundwater inflow. This study compares the allowable lateral load capacity (LLC) of plain-reinforced-plain (PRP) secant piles with single piles through model tests. The behavior of both PRP secant piles and single piles under lateral loading was investigated for various soil types (sand and clay) and pile diameters (21, 28, and 36 mm). Lateral load–displacement curves derived from experimental study were used to assess the allowable LLC and displacement characteristics of both pile types. The results highlighted a marked difference in the allowable LLC of PRP secant piles over single piles, particularly in sand, with their allowable LLC being approximately 2.10 to 3.40 times greater, and 1.40–1.90 times higher in clay. The increase in pile diameter also improved the allowable LLC for both pile types under different soil conditions. These findings highlight the superior performance and flexibility of PRP secant piles under weak ground conditions and provide valuable insights for design optimization and construction practices.