Semi‐brittle flow occurs when crystal plasticity and cataclastic mechanisms operate concurrently and may be common at mid‐levels of the Earth's crust. Using a Paterson gas‐deformation apparatus, we performed 67 conventional triaxial deformation experiments on dry samples of Carrara marble in the temperature range of T = 20–800°C; confining pressures (PC) were 30, 50,100, 200, and 300 MPa, and strain rates (ε˙ $\dot{\varepsilon }$) were 10−3, 10−4, 10−5, and 10−6 s−1. Axial strains (ε) were ≲0.12. The measured (differential) stress, Δσ=σ1−PC ${\Delta}\sigma ={\sigma }_{1}-{P}_{\mathrm{C}}$, changes with strain at most applied conditions. At ε = 0.05, both stress and the hardening coefficient (h), that is, the rate of increase of stress with strain, increase as T decreases and PC increases. At T ≲ 400°C, h is quite large and the sensitivity of Δσ ${\Delta}\sigma $ on ε˙ $\dot{\varepsilon }$ is low, while both are sensitive to increasing pressure. In this temperature range, the mechanical behavior of the marble is very similar to that exhibited by high‐strength, high‐ductility, hexagonal metals that deform by processes called twinning induced plasticity (TWIP). Twinning and dislocation motion are abundant in the samples, as are inter‐ and intracrystalline microfractures. The concurrent activation of these deformation mechanisms leads to complex relationships of Δσ ${\Delta}\sigma $ and h with the applied T ‐ PC‐ε˙ $\dot{\varepsilon }$ conditions. This behavior suggests that peak stresses for calcite rocks deforming by semi‐brittle processes will occur at PC‐ T conditions of the middle crust, and that they might be more strongly influenced by total strain rather than by strain rate. Plain Language Summary: Natural deformation of rocks at shallow depth is expected to be dominated by (micro‐) fracturing, whereas deeper within the Earth's crust, crystal‐plastic mechanisms probably dominate. Semi‐brittle flow, which involves the concurrent operation of both sets of deformation mechanisms, is probably common in the middle crust. To better understand this mechanical behavior, we deformed Carrara marble samples over a wide range of temperatures, pressures, and strain rates. The stress supported by the marble changes as ineleastic strain increases and generally increases with increasing pressure and decreasing temperature at a given strain. Strain rate had a small influence on stress at temperatures ≲400°C. This mechanical behavior is quite comparable to that of some hexagonal metals, which deform by a process called twinning induced plasticity at temperatures below about half the melting point. The microstructure of the marble samples shows that three deformation mechanisms are operating: Twinning, dislocation motion, and microfracturing. The relative intensity of these mechanisms depends on deformation conditions. The correlations between applied conditions and mechanical response that we observed suggests that strength of calcite rocks in the middle crust is indeed governed by semi‐brittle processes and depends more on total strain and less on variations of strain rate. Key Points: The mechanical behavior and microstructure evolution of marble was studied in triaxial compression tests over a wide range of conditionsSemi‐brittle deformation occurred by strain partitioning amongst cataclasis, twinning, and dislocation motion over all conditionsAt temperatures between ≈200 and 400°C, changes in strain rate and pressure had reduced influence on strength and hardening [ABSTRACT FROM AUTHOR]