Finite field multiplication over GF(2m) is one of the most important arithmetic operations for Elliptic Curve Cryptosystem (ECC). Polynomial basis multipliers over GF(2m) are widely applied in ECC due to its regular, modular, easily expansible benefits and the high suitability for VLSI implementation. This study will present a novel digit-serial polynomial basis multiplier using Karatsuba algorithm representation. To achieve efficient architectures, our proposed digit-serial architecture is different from existing digit-serial polynomial basis multipliers that use cut-set algorithm. The proposed digit-serial polynomial basis multiplier saves 90% space complexity as compared to existing similar studies. Existing digit-serial polynomial basis multipliers employ one dimensional array of digit cells, but our proposed digit-serial polynomial basis multiplier uses only one digit cell.