Triple X syndrome (TXS, also known as trisomy X or 47,XXX) has been associated with impaired overall neurocognitive functioning in children and relatively young adults. However, neurocognitive functioning in adults with TXS is poorly understood. The aim of this study was, therefore, to examine cognitive functioning in adults with TXS. Methods: In this cross-sectional study, data were collected from 34 adult women with TXS (mean age = 32.9; SD = 13.1) and 31 controls (mean age = 34.9; SD = 13.7). General intellectual functioning, semantic/verbal memory, visual/episodic memory, psychomotor speed, and attention and executive functioning were then compared between these two groups. Results: We found that general intellectual functioning was significantly lower in the TXS group compared to the control group. In addition, women with TXS had more attention problems and lower psychomotor speed, particularly motor processing speed. When the analyses were adjusted for IQ, the strength of these associations decreased. The women in the TXS group also scored significantly lower at free recall in the verbal memory test, but not in immediate or delayed recognition. Finally, visual/episodic memory and executive functioning did not differ significantly between groups. Conclusions: Our analysis revealed that women with TXS score lower in general intellectual functioning and have impairments in motor processing speed and attention compared to controls, but do not differ with respect to executive functioning. These results offer new insights for improving the support of adults with TXS both at school and in the workplace.