Gelidium maggsiae sp. nov. is described from plants found in shallow subtidal maërl beds off the west coast of Ireland, where it attaches to detached coralline algae by peg-like rhizoids. Rhizines are more abundant in the medullary region than in the cortex and erect axes display a prominent central row formed by the axial and periaxial filaments. Gametophytes are bisexual and protandrous and were found only in culture. Post-fertilization and cystocarp development are similar to that of G. pteridifolium and cystocarps are biloculate. Tetrasporophytes are morphologically similar to gametophytes; tetrasporangia are irregularly arranged in pod-shaped terminally inserted stichidia, borne on erect axes and arranged unilaterally. Gametangial plants reproduced under short-day (8[ratio ]16 h) conditions at 20–25°C in enriched media, but gametangial formation was independent of photoperiod when plants were kept in unenriched medium. No tetrasporophytic plants were produced from germinated carpospores, but plants originally collected in a vegetative state formed tetraspongia at 20°C, 16[ratio ]8 h. The validity of several characters previously used in species delimitation within the Gelidiales is discussed, and only cystocarp structure and the bisexual nature of the gametophyte are considered to be reliable characters.