Rehydrating maize (Zea mays L.) grain silage can mitigate storage issues in maize production. Moreover, the use of agricultural byproducts for rehydration may reduce environmental impact and prevent aerobic deterioration of maize grain silage. This study investigated the effects of increasing levels of wet tomato byproduct (WTB) on the fermentation characteristics, nutritional value, and aerobic stability of maize grain silage. A completely randomised design with five treatments and seven replicates was applied. Treatments consisted of ground maize grains (1–2 mm) replaced with increasing WTB levels (25%, 30%, 35%, 40%, and 45%) in the ensiled mass, added based on a fresh mass basis. A linear decrease (P < 0.01) in dry matter (DM) content from 77.6% to 53.4% was observed. Crude protein, neutral detergent fibre, and lignin concentrations increased linearly (P < 0.01), with respective increments of 4.13%, 4.72%, and 1.14% between 25% and 45% WTB additions. A negative quadratic effect was found in the in vitro digestible DM content (P < 0.01), with the highest value at 30% WTB (75.6%). Lactic acid concentrations also rose, from 1.01% to 7.35%, with increasing WTB concentrations. No aerobic deterioration was observed after 264 h of air exposure in the silages supplemented with 35%, 40%, and 45% WTB. However, excessively high WTB proportions reduced DM content and increased the concentration of indigestible fibre. Rehydrating maize grain silage with 35–40% WTB improved fermentation and aerobic stability without compromising digestibility.