In line with the recent promotion of biocementation as an environmentally friendly ground improvement method, this study presents an investigation into microbially induced calcite precipitation (MICP) as a method of improving the engineering properties of soft clay. Bacillus pasteurii bacterium in vegetative cell and bacterial spore forms were used to induce MICP in clay specimens. Untreated and treated clay specimens were tested for their mechanical properties and microstructures through unconfined compression (UC) tests, free-free resonance (FFR) tests, X-ray diffraction (XRD) tests, and scanning electron microscopy with energy dispersive X-ray (SEM/EDX) tests. Results showed that both vegetative cells and bacterial spores can effectively enhance the strength and modulus of clays by inducing MICP to generate calcite crystals. Clays treated with vegetative cells exhibited earlier improvements in their strength than clays treated with bacterial spores due to earlier activity availability; however, the clays treated with bacterial spores exhibited greater strength improvements in the long term. Bacterial spores may also prove more convenient to use in geotechnical engineering practice.