The photonic terahertz integrated sensing and communication (THz-ISAC) holds immense potential for enabling ultrahigh data rates and millimeter-resolution sensing, due to its exceptionally broad bandwidth. Nevertheless, the limited radiation power of existing THz sources restricts the full utilization of their broadband advantages in single-channel ISAC systems. In this work, a photonic THz-ISAC system model utilizing the chirp spread spectrum (CSS) waveform scheme is proposed and theoretically analyzed, by considering the noise contribution of the photonic THz transceiver and the free-space propagation loss (FSPL). Based on the proposed waveform, we derive the closed form of communication error vector magnitude (EVM) and radar sensing Cramér–Rao lower bound (CRLB), as well as the performance bounds. Then, a proof-of-concept experiment operating at 285 GHz is conducted to verify the performance of the CSS waveform, achieving 12 Gbit/s transmission with a 1.5 cm range resolution. In addition, the experiment validates the performance trade-offs among chirp bandwidth, data rate, and signal power, aligning well with our theoretical bounds. Therefore, the proposed scheme provides a design guideline for CSS-based photonic THz-ISAC systems.