Optical modulators hold great promise for nonlinear optical and photonic devices. However, current optical modulators are exclusively based on inorganic saturable absorbers such as semiconductor thin films, 2D materials, and plasmonic nanocrystals. Here, nonlinear optical modulation is demonstrated by designing a series of organic semiconducting polymer dots (Pdots) for passive Q-switching and mode-locking lasers in the near-infrared (NIR) region. Combining density functional theory and judicious polymer design, Pdots with large extinction coefficient and broadband absorption are developed which span the entire spectrum from ultraviolet (UV) to NIR region (300 to 2500 nm). The Pdots exhibit strong saturable absorption (β = −1.2 × 10⁻³ cm W⁻¹) and optical nonlinearity (Imχ⁽³⁾ = −8.4 × 10⁻⁷ esu), which are of orders higher than other nonlinear optical materials such as carbon-based materials, plasmonic metal nanoparticle, and semiconductor nanocrystals. The Pdot modulators are integrated into the fiber laser systems, and demonstrate passively mode-locked lasers at 1036, 1038, 1945, 1950 nm and Q-switched lasers at wavelengths of 1559 and 1957 nm, highlighting the potential of Pdots for advanced nonlinear optical and photonic applications.