We prove a "splicing formula" for the LMO invariant, which is the universal finite-type invariant of rational homology 3-spheres. Specifically, if a rational homology 3sphere M is obtained by gluing the exteriors of two framed knots K1 ⊂ M1 and K2 ⊂ M2 in rational homology 3-spheres, our formula expresses the LMO invariant of M in terms of the Kontsevich-LMO invariants of (M1, K1) and (M2, K2). The proof uses the techniques that Bar-Natan and Lawrence developed to obtain a rational surgery formula for the LMO invariant. In low degrees, we recover Fujita's formula for the Casson-Walker invariant and we observe that the second term of the Ohtsuki series is not additive under "standard" splicing. The splicing formula also works when each Mi comes with a link Li in addition to the knot Ki, hence we get a "satellite formula" for the Kontsevich-LMO invariant. Contents 1. Introduction 1 2. Splicing and surgery 3 3. Splicing formula 7 4. Low degree formulas 13 5. Splicing and satellite operations 16 6. The case of knots that are not trivial in homology 19 Appendix A. Signatures of tridiagonal matrices 22 References 25