We prove that every Diophantine quadruple in R[X] is regular. More precisely, we prove that if {a, b, c, d} is a set of four non-zero polynomials from R[X], not all constant, such that the product of any two of its distinct elements increased by 1 is a square of a polynomial from R[X], thenOne consequence of this result is that there does not exist a set of four non-zero polynomials from Z[X], not all constant, such that a product of any two of them increased by a positive integer n, which is not a perfect square, is a square of a polynomial from Z[X]. Our result also implies that there does not exist a set of five non-zero polynomials from Z[X], not all constant, such that a product of any two of them increased by a positive integer n, which is a perfect square, is a square of a polynomial from Z[X].