Teacher workload is an important policy concern in many education systems around the world, often considered a contributory factor in teacher attrition. One aspect of workload that could be addressed is reducing the amount of written marking and feedback that teachers do. This article reports on the results of an evaluation of FLASH Marking, an intervention aimed at reducing teachers' marking workload. FLASH Marking is a code-based feedback approach involving peer- and self-assessment, reducing the need to use alphanumeric grading while promoting the use of students' metacognitive skills. The study involved a single cohort of 18,500 Key Stage 4 pupils (aged 14/15 at the start of the trial) and their English teachers (n = 990) in 103 secondary schools in England. The impact of the intervention was estimated as the difference in before and after measures of teacher workload, comparing teachers in 52 intervention schools and those in 51 control schools. The results suggest that the intervention had the effect of lessening teachers' workload by reducing their working hours (effect size 0.16), including hours spent on marking and feedback (0.17). The intervention was largely implemented as designed and teachers were generally positive about the potential impact of FLASH on pupils' learning outcomes.