Background: The genus Amaranthus is taxonomically complex because of its high morphological variability, which led to nomenclatural confusion, misapplication of names and misidentifications. Unfortunately, floristic and taxonomic studies on this genus are still incomplete. A population of Amaranthus crassipes subsp. warnockii was discovered in Monastir Governorate, Tunisia, representing the first record for both the Tunisian and the African floras, as well as the first one outside of its American native distribution area. Objectives: The main aim of the present study was to record Amaranthus crassipes subsp. warnockii (I.M.Johnst.) N.Bayón in Tunisia and Africa for the first time. Morphological characters and ecological data were provided. Clarification about the typification of the names Amaranthus crassipes, A. warnockii and Scleropus amaranthoides was also presented. Methods: The work was based on field surveys, analysis of relevant literature and examination of specimens preserved in the herbaria GH, HAL, P, RO, NY, US and the Herbarium of the Faculty of Pharmacy of Monastir (Monastir University). Results: Nomenclatural notes were provided for Schlechtendal’s A. crassipes (lectotype at HAL, designated by Henrickson in 1999 but here corrected according to Art. 9.10 of the ICN); Schrader’s Scleropus amaranthoides [a superfluous and illegitimate name (Arts. 52.1 and 52.2 of the ICN) and regarded as a homotypic synonym of A. crassipes s.str.]; and Moquin-Tandon’s Scleropus amaranthoides [an invalid name (Art. 36.1a of the ICN)]. Conclusion: Amaranthus crassipes subsp. warnockii is an alien species in Tunisia, growing in human-made habitat on clayey and sandy substrates within ruderal vegetation where it can be considered a casual. We hope that in the future continuous monitoring of the population will take place, to verify the possible naturalisation and spread of this taxon in Tunisia. If the latter happens, actions for eradication of the plants are needed.