Brassica napus L. is one of the most important oil crops in the world, and improving its nitrogen efficiency is a key strategy for enhancing low-nitrogen tolerance and increasing yield. Ammonium transporters (AMTs) play critical roles in NH₄⁺ transport in plants and significantly influence nitrogen use efficiency and yield. To identify promising candidate AMT genes for rapeseed improvement, this study systematically analyzed the evolution and expression of AMT genes across the Archaeplastida plants, especially in Brassica species of the U’s triangle. A total of 368 AMT genes were identified from 35 Archaeplastida species, including 180 AMT1 and 188 AMT2 homologs. Gene duplication analysis revealed that small-scale duplication (SSD) was the primary mechanism driving gene expansion from aquatic algae to angiosperms in both AMT1 and AMT2 families, which were under the influence of purifying selection. In the U’s triangle, allopolyploidization, particularly whole-genome duplication (WGD), was the primary mechanism for AMT expansion, accompanied by gene loss. Expression profiling indicated that most BnAMTs respond to hormonal and low-nitrogen signals and exhibit tissue-specific expression patterns. Notably, four candidate genes (BnAMT1;5, BnAMT1;12, BnAMT2;2, BnAMT2;4) exhibited high expression levels and responsiveness to low-nitrogen stress. Haplotype analysis further linked specific polymorphisms in BnAMT1;5 and BnAMT2;2 to variations in plant height and yield. Collectively, these findings elucidate the evolutionary processes and regulatory mechanisms of AMTs, laying a foundation for enhancing nitrogen utilization efficiency in B. napus and other crops.