Solar activity stands as a pivotal focus in solar physics research, and is the source of space weather that can profoundly impact the Earth’s environment and our daily lives. Thanks to remarkable advancements in both space-borne and ground-based observational techniques and facilities, numerical simulations and machine learning techniques, we have witnessed fruitful achievements across various facets of in the ongoing 25th solar cycle. As the cycle has seen its first (possibly the only) peak, we initiated a Special Issue entitled ‘Solar Activity During Cycle 25‘ in the Universe Journal (IF 2.6; JCR-Q2) published by MDPI, which aims to showcase the latest research findings along with relevant theoretical modeling efforts and instrumental advancements. Potential topics include, but are not limited to: 1) Solar dynamo and the solar cycle; 2) Solar magnetic field and its role in solar activity; 3) Solare flares: energy accumulation and release, triggers, dynamics, particle acceleration and hard X-ray (HXR) emission; 4) Coronal mass ejections (CMEs): source region, acceleration and dynamical evolution and propagation; 5) Solar activity and space weather: modeling, prediction and impact on the Earth; 6) Chromospheric and coronal heating: observational insights and simulation studies; 7) Machine learning applications in the diagnosis and analysis of solar activity; 8) Current and upcoming solar instrumentations.