Gaertnera Lam. (Rubiaceae, Gaertnereae) includes 42 species confirmed from Madagascar, with 16 of them newly described here as a result of recent botanical exploration. The Madagascar species of Gaertnera have notable morphological variation. Newly described, illustrated, and keyed here are G. arenarioides C. M. Taylor, G. breviflora C. M. Taylor, G. hirsuta C. M. Taylor, G. laevis C. M. Taylor, G. littoralis C. M. Taylor, G. malcomberiana C. M. Taylor, G. masoalana C. M. Taylor, G. nitida C. M. Taylor, G. rakotovaoana C. M. Taylor, G. razakamalalana C. M. Taylor, G. robusta C. M. Taylor, G. rubra C. M. Taylor, G. sclerophylla C. M. Taylor, G. velutina C. M. Taylor, G. vernicosa C. M. Taylor, and G. xerophila C. M. Taylor. Narrower circumscriptions of G. obovata Baker var. sphaerocarpa (Baker) Malcomber and G. phanerophlebia Baker here are detailed by revised descriptions. Assessments of conservation status of Gaertnera in Madagascar using IUCN categories find two species Critically Endangered, 10 Endangered, 14 Vulnerable, two Near Threatened, and 14 species and two varieties of Least Concern.