Herein, we describe the hydrothermal synthesis of a novel cobalt-seamed C -ethylpyrogallol­[4]­arene-based metal–organic nanocapsule (Co 24 PgC 2 MONC) that can be easily converted into micelles in water using Tween-20, a natural polysorbitan. Such micellar forms exhibit increased efficiency up to 16-fold for photocatalytic hydrogen evolution with a high rate of 7.2 mmol g –1 h –1 outperforming Co 24 PgC 2 MONC without Tween-20. Notably, Tween-20 is critical in this system because it can homogenize the heterogeneous Co 24 PgC 2 MONC catalyst, provide numerous accessible active sites and inhibit the energy transfer between photosensitizer and nanocapsules. This investigation not only provides a new approach to study a water-insoluble coordination complex but also offers an effective strategy to increase their hydrogen evolution catalytic activities.