We performed interspecific hybridization in the haploid blade-forming marine species (nori) of the genus Porphyra, which have a heteromorphic life cycle with a haploid gametophytic blade and a diploid microscopic sporophyte called the “conchocelis phase.” The green mutant HGT-6 of P. tenera var. tamatsuensis A. Miura was crossed with the wildtype HG-1 of P. yezoensis f. narawaensis A. Miura; the F1 heterozygous conchocelis developed normally and released numerous conchospores. However, almost all the conchospore germlings did not survive past the four-cell stage or thereabouts, and only a few germlings developed into gametophytic blades. These results indicate that hybrid breakdown occurred during the meiosis, while the surviving F1 gametophytic blades were considered a breakthrough in the interspecific hybridization of Porphyra. Organelle genomes (cpDNA and mtDNA) were found to be maternally inherited in the interspecific hybridization by molecular analyses of the organelle DNA. In particular, molecular analyses of nuclear DNA revealed that the surviving F1 blades were allodiploids in the haploid gametophytic phase; however, there is a possibility of the occurrence of rapid chromosomal locus elimination and rearrangement in the F1 conchocelis phase. Our findings are noteworthy to the breeding of cultivated Porphyra and will provide important information for understanding of the speciation of marine plants with high species diversity.