ABSTRACT Background Workplace-Based Assessment (WPBA) forms a major component of the UK General Practitioner (GP) licensing, together with knowledge and clinical skills examination. WPBA includes Case-based Discussion, Consultation Observation Tool, Mini-Consultation Exercise, Multisource Feedback, Patient Satisfaction Questionnaire, Clinical Examination and Procedural Skills, Clinical Supervisor’s Report, and Educational Supervisor Review. We aimed to investigate GP trainees’ and trainers’ perceptions and experiences of WPBA regarding validity and fairness. Methods We used a national online survey, with Likert-scaled and free-text responses, to a convenience sample of GP trainees and trainers, on perceptions and experiences of WPBA. Analysis included descriptive statistics, scale development, and regression models to investigate factors associated with attitudes towards WPBA, with thematic analysis of free text responses supported by NVivo 12. Results There were 2,088 responses from 1,176 trainees and 912 trainers. Both groups were generally positive towards WPBA, with trainers more positive or similar to trainees towards individual assessments. In a multivariable regression model, accounting for sex, ethnicity and country of primary medical qualification, trainees were significantly less positive (p < 0.001) while international medical graduates (IMGs) trained outside the European Economic Area (EEA) were significantly more (p < 0.001) positive towards WPBA. Qualitative analysis revealed varying concerns about validity and relevance, assessment burden, potential for bias, fairness to protected characteristics groups, gaps in assessment, and perceptions of individual assessments. Discussion Trainers’ greater positivity towards elements of WPBA accords with their role as assessors. Despite concerns about bias, IMGs from outside the EEA were significantly more positive towards WPBA.