High speed, high resolution and high sensitivity are desirable for optical coherence tomography (OCT). Here, we demonstrate a spacedivision multiplexing (SDM) technology that translates long coherence length of a commercially available wavelength tunable laser into high OCT imaging speed. We achieved an effective 800,000 A-scans/s imaging speed using a 100,000 Hz tunable vertical cavity surface-emitting laser (VCSEL). A sensitivity of 94.6 dB and a roll-off of < 2 dB over ̃30 mm imaging depth were measured from a single channel in the prototype SDM-OCT system. An axial resolution of ̃11 μm in air (or ̃8.3 μm in tissue) was achieved throughout the entire depth range. An in vivo, 3D SDM-OCT volume of an entire Drosophila larva consisting of 400 x 605 A-scans was acquired in 0.37 seconds. Synchronized cross-sectional OCT imaging of three different segments of a beating Drosophila larva heart is demonstrated. The SDM technology provides a new orthogonal dimension for further speed improvement for OCT with favorable cost scaling. SDMOCT also preserves image resolution and allows synchronized crosssectional and three-dimensional (3D) imaging of biological samples, enabling new biomedical applications.