Summary: We investigate an integrable extended modified Korteweg-de Vries equation on the line with the initial value belonging to the Schwartz space. By performing the nonlinear steepest descent analysis of an associated matrix Riemann-Hilbert problem, we obtain the explicit leading-order asymptotics of the solution of this initial value problem as time \(t\) goes to infinity. For a special case \(\alpha=0\), we present the asymptotic formula of the solution to the extended modified Korteweg-de Vries equation in region \(\mathcal{P} = \{(x,t) \in \mathbb{R}^2 \mid 0 < x \leq Mt^{\frac{1}{5}}, \,t \geq 3 \}\) in terms of the solution of a fourth order Painlevé II equation.