Stable embedded solitons are discovered in the generalized third-order nonlinear Schrödinger equation. When this equation can be reduced to a perturbed complex modified KdV equation, we developed a soliton perturbation theory which shows that a continuous family of sech-shaped embedded solitons exist and are nonlinearly stable. These analytical results are confirmed by our numerical simulations. These results establish that, contrary to previous beliefs, embedded solitons can be robust despite being in resonance with the linear spectrum. [PACS: 42.65.Tg, 05.45.Yv.] Pulse propagation in optical fibers attracts a lot of attention these days. Pico-second pulses are well described by the nonlinear Schrödinger (NLS) equation which accounts for the second-order dispersion and self-phase modulation. But for femtosecond pulses, other physical effects such as the third-order dispersion and selfsteepening become non-negligible. The physical model which incorporates these additional effects is [1,2] (1)Here ψ is the envelope of the electric field, z is the distance, τ is the retarded time, β and µ are the selfsteepening coefficients [1]. All quantities have been normalized. For optical pulses, β = µ [1,2]. In this paper, we allow β and µ to be different for the sake of mathematical analysis. The Raman effect, which is dissipative in nature, is also non-negligible [1,2]. It is not included in the model (1) because we want to focus our attention on the other physical effects in this paper. The third-order dispersion term in Eq.(1) is significant because it qualitatively changes the linear dispersion relation of Eq. (1). Its effect on the NLS soliton is to generate continuous-wave radiation and causes the soliton to decay [1][2][3][4][5]. However, solitary waves of Eq. (1) which are embedded inside the linear spectrum do exist in certain parameter regimes [6][7][8][9], and such waves are called embedded solitons [10,11]. To see why a solitary wave of Eq. (1) has to be an embedded soliton, we substitute solitary waves of the form Ψ(τ − V z)e iλz into (1), where velocity V and frequency λ are constants. We readily find that for any frequency λ, the linear equation for Ψ allows oscillatory solutions. Thus, all λ lies in the continuous spectrum of Eq. (1), hence the solitary wave must be an embedded soliton. Stability of these embedded solitons is clearly an important issue. Previous analytical studies have shown that if embedded solitons are isolated in a conservative system, they are at most semi-stable, i.e., the perturbed soliton persists or decays depending on whether the initial energy is higher or lower than that of the embedded soliton [10,12,13]. A physical explanation for it is as follows [10]. If the initial state has energy higher than the embedded soliton, it just sheds extra energy through tail radiation and asymptotically approaches this embedded soliton; but if the initial state has lower energy than the embedded soliton, the energy loss (through radiation) drives the solution away from the embedded soliton, re...