In this paper, we propose a generalized-chirp spread spectrum (G-CSS) that can have various modulation orders and offers superior bit error rate (BER) performance to improve the communication performance of existing chirp-based modulation/demodulation schemes. The proposed G-CSS sets frequency bins with different modulation orders at the start and end points of the symbol and selects frequency bins based on the bits to be transmitted, modulating the signal using corresponding chirps. Therefore, the proposed method allows for independent design of modulation orders at the start and end of the symbol, enabling signal design and flexible transmission tailored to the transmission rate required by the system. Through computer simulations and practical ocean experiments, we compared and analyzed the BER performances of the proposed G-CSS with existing up/down-CSS and long-range CSS (LoRa-CSS). The proposed G-CSS demonstrated a superior BER performance at the same transmission rate.