Avibacterium paragallinarum has been subtyped into three serogroups (A, B, and C) and nine serovars (A-1, A-2, A-3, A-4, B-1, C-1, C-2, C-3, and C-4) according to the Page and Kume schemes. Both schemes use the hemagglutination inhibition test for serotyping. However, the relationship between the hemagglutinin gene (HMTp210) sequences and serotypes of A. paragallinarum is still unclear. This problem is partly due to the lack of information on the complete HMTp210 sequence from the formal reference strain of Page serogroup B (strain 0222 or Spross). In this study, we determined the complete HMTp210 sequence of strain Spross. The sequence of Spross and those of other HMTp210 sequences retrieved from GenBank were used to conduct phylogenetic analyses to investigate the relationship between the serotypes and HMTp210 sequences of A. paragallinarum. Four phylogenetic clusters, designated clusters A-1, A-2, B, and C, were identified. Clustering based on complete HMTp210 sequences correlates with serotyping based on hemagglutination inhibition tests. Serovar A-2 was found to contain a chimeric HMTp210 gene that might have resulted from recombination between serovar A-1 and serovar C-1. In addition, phylogenetic analysis based on partial sequences (approximately nucleotides 1–1200) of HMTp210 was sufficient to discriminate between serogroups A, B, and C. These findings could be valuable for developing a molecular method for serotyping of A. paragallinarum.