A modular shelving system comprises multiple panels, multiple connection bars and multiple connectors. Each panel has multiple mortises. Each connection bar is connected to at least two panels and has multiple housing channels having slots. The connectors are respectively mounted in the housing channels of the connection bars and have four shoulders and multiple arms. The shoulders are formed at right angles to each other and one shoulder is mounted in a corresponding slot. The arms are formed on, protrude from the shoulders and respectively extend into corresponding mortises of the panels and two, three or four arms may be implemented. The modular shelving system can be conveniently assembled and disassembled in various combinations for different spaces while maintaining design consistency.