The infrageneric classification of the cosmopolitan genus Lobelia L. (Campanulaceae) is revised via synthesis of diverse phenotypic data. The 415 species are divided among 18 sections (numbers of included species are given in parentheses): Lobelia sect. Delostemon (E. Wimm.) J. Murata (44); Lobelia sect. Holopogon Benth. (14); Lobelia sect. Colensoa (Hook. f.) J. Murata (1); Lobelia sect. Cryptostemon (E. Wimm.) J. Murata (10); Lobelia sect. Stenotium (C. Presl) Lammers (144); Lobelia sect. Lobelia (22); Lobelia sect. Hypsela (C. Presl) Lammers (43); Lobelia sect. Mezleriopsis Lammers, sect. nov. (7); Lobelia sect. Jasionopsis Lammers, sect. nov. (1); Lobelia sect. Tylomium (C. Presl) Benth. (38); Lobelia sect. Homochilus A. DC. (5); Lobelia sect. Tupa (G. Don) Benth. (4); Lobelia sect. Trimeris (C. Presl) A. DC. (1); Lobelia sect. Speirema (Hook. f. & Thomson) Lammers (5); Lobelia sect. Plagiobotrys Lammers (1); Lobelia sect. Rhynchopetalum (Fresen.) Benth. (61); Lobelia sect. Revolutella E. Wimm. (9); and Lobelia sect. Galeatella E. Wimm. (5). In addition, Monopsis Salisb. sect. Xanthomeria (C. Presl) Lammers, comb. nov., is validated to provide the correct name for Monopsis sect. Parastranthus (G. Don) E. Wimm.