We demonstrate all-optical ultrahigh-speed swept-source optical coherence tomography (OCT) based on amplified optical time-stretch (AOT). Such an inertia-free wavelength-swept mechanism, via group velocity dispersion, enables us to realize OCT with an A-scan rate well above MHz. More importantly, the key significance of AOT-OCT is its simultaneous broadband Raman amplification during the time-stretch process-greatly enhancing the detection sensitivity compared with prior attempts to apply optical time-stretch to OCT. Here, we report on an AOT-OCT system which is operated at an A-scan rate of 7.14 MHz, a superior roll-off performance (>2 mm/dB), a record-high sensitivity of time-stretch-based OCT (>80 dB) with a broadband gain bandwidth of 80 nm, which results in an axial resolution of ∼15 μm. Our AOT-OCT system is thus able to, for the first time to the best of our knowledge, perform time-stretch-based OCT of biological tissue in vivo. It represents a major step forward in utilizing AOT as an alternative for achieving practical MHz OCT, without any long-term mechanical stability concerns as in typical swept-source OCT or bypassing the speed limitation of the image sensor employed in spectral-domain OCT.