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The brown macroalga <i>Saccharina japonica</i> (Laminariales, Phaeophyta) is the most productive cultured seaweed in the world. In order to improve the biosafety of transgenic kelp, it is necessary to develop endogenous constitutive promoters, replacing those of virus origin. In this study, the housekeeping actin gene from <i>S. japonica</i> (<i>SjACT</i>) was found to contain three exons and two introns, representing a unique actin gene structure pattern in brown algae. Additionally, the 5′ upstream region was obtained using genome walking, and fused to the reporter gene <i>lacZ</i> or <i>EGFP</i> to construct promoter-detective vectors. Using an established genetic transformation system, kelps in different life-cycle stages were transformed. The detection results showed that, in the diploid sporophyte stage, the transient expression from the <i>lacZ</i> gene could be observed in the frond, stipe, or holdfast of kelps, indicating a manner of being non-tissue-specific. And, in the haploid gametophyte stage of <i>S. japonica</i>, the fluorescence of the expressed EGFP were detected in vivo in gametophyte cells of both genders. These results indicate that the promoter of the <i>SjACT</i> gene (p<i>SjACT</i>) functions in a constitutive manner and is expected to be a key endogenous element in the genetic manipulation of kelps.