Background: Cognitive impairments are well-established features of schizophrenia whereas there is ongoing debate about nature and degree of cognitive performance in schizoaffective disorder and bipolar disorder. We hypothesised that there is a spectrum of increasing impairment from bipolar disorder to schizoaffective-bipolar type to schizoaffective-depressive type and schizophrenia. Method: Performance on the MATRICS Consensus Cognitive Battery was compared between participants with schizophrenia (N=558), schizoaffective-depressive type (N=112), schizoaffective-bipolar type (N=76), bipolar disorder (N=78) and healthy participants (N=103) using analysis of covariance with post-hoc comparisons. An ordinal logistic regression was conducted to examine whether cognitive impairments followed the hypothesised spectrum from bipolar disorder (least severe) to schizophrenia (most severe). In addition to categorical diagnoses we addressed the influence of symptom domains, examining the association between cognition and mania, depression and psychosis. Results: Cognitive impairments increased in severity from bipolar disorder to schizoaffective-bipolar to schizophrenia/schizoaffective-depressive. Participants with schizophrenia and schizoaffective-depressive displayed equivalent performance (d=0.07,p=0.90). The results of the ordinal logistic regression were consistent with a spectrum of deficits from bipolar disorder to schizoaffective-bipolar type to schizophrenia/schizoaffective-depressive type (OR=1.98, p=2.4x10-16). In analyses of the associations between symptom dimensions and cognition, higher scores on the psychosis dimension were associated with poorer performance (B=0.015 p=3.2 x 10-16). Limitations: There were fewer participants with schizoaffective disorder and bipolar disorder than schizophrenia. Despite this, our analyses were robust to differences in the group sizes and we were able to detect differences between groups. Conclusion: Cognitive impairments represent a symptom dimension that cuts across traditional diagnostic boundaries.