Aims To estimate the changes in intraocular pressure (IOP) during the first 24 h after transscleral cyclophotocoagulation (TCP). Methods A prospective single‐centre study, where patients with glaucoma destined for treatment with TCP were asked for participation. The IOP was measured prior to TCP and at 1, 2, 4, 6 and 24 h post‐TCP. An IOP spike was defined as an elevation of IOP of ≥5 mmHg compared with baseline. The visual acuity (VA) was examined at baseline and after 24 h. Results The mean IOP prior to TCP in 58 eyes of 58 patients was 26.2 (±8.9 SD) mmHg. Twenty‐three eyes (40%) experienced an IOP spike at some examination timepoint during the first 24 h. The mean value of the IOP spike was 12.1 (±6.9) mmHg. Fifty‐six per cent of the eyes with pseudoexfoliation glaucoma (PEXG) experienced an IOP spike, and 16% had an IOP spike ≥20 mmHg. The IOP was significantly reduced at the 24 h examination by 8.1 (±7.8) mmHg (n = 58). The VA 24 h after TCP was unchanged compared with baseline. Conclusion Clinically significant IOP spikes were common in the first 24 h post‐TCP. Almost one in five eyes had an increase of 10 mmHg and in almost one in 10 eyes, the IOP increase was 20 mmHg or higher. Eyes with PEXG had a higher occurrence of IOP spikes and displayed a greater magnitude of IOP elevation. Prophylactic post‐operative IOP‐lowering medication should be considered to prevent further glaucoma damage.