A self-locking connection system for modular construction (e.g., MiC and PPVC) is provided for interlocking an upper module column to a lower module column. A horizontal load transfer plate has first and second inner sleeve portions positioned beneath and above the plate. The sleeves are configured and dimensioned to be received within the respective module columns. Spring-loaded latches in both sleeve portions engage respective column receiving apertures. Each latch may include a latch plate having a wedge-shaped latch protrusion connecting to a vertical latch surface. The latch plate has one or more latch plate apertures for receiving a rod within a coil spring. An optional second reversible self-locking mechanism interlocks the connected modules to a building load-bearing support such as a core wall. The second self-locking mechanism includes an angled protrusion extending from the horizontal load transfer plate to mate with a protrusion-receiving structure embedded in the load-bearing support.