We prove that, if M>4false(123false) and ɛ > 0, if V and W are complex JBW*‐triples (with preduals V* and W*, respectively), and if U is a separately weak*‐continuous bilinear form on V × W, then there exist norm‐one functionals ϕ1, ϕ2 ∈ V* and ψ1, ψ2 ∈ W* satisfying false|Ufalse(x,yfalse)false|⩽Mthinmathspace∥U∥false(∥x∥φ22ε2thinmathspace∥x∥φ12false)12false(∥y∥ψ22ε2thinmathspace∥y∥ψ12false)12 for all (x, y) ∈ V × W. Here, for a norm‐one functional ϕ on a complex JB*‐triple V, |·|ϕ stands for the prehilbertian seminorm on V associated to ϕ given by falsefalse∥x∥φ2:=φfalsefalse{x,x,zfalsefalse} for all x ∈ W, where z ∈ V** satisfies ϕ z = |z| = 1. We arrive at this form of ‘Grothendieck's inequality’ through results of C.‐H. Chu, B. Iochum, and G. Loupias, and an amended version of the ‘little Grothendieck's inequality’ for complex JB*‐triples due to T. Barton and Y. Friedman. We also obtain extensions of these results to the setting of real JB*‐triples. 2000 Mathematical Subject Classification: 17C65, 46K70, 46L05, 46L10, 46L70.