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We report the discovery of a close-in ($P_{\mathrm{orb}} =3.349\:\mathrm{days}$) warm Neptune with clear transit timing variations (TTVs)orbiting the nearby ($d=47.3\:\mathrm{pc}$) active M4 star, TOI-2015. Wecharacterize the planet's properties using TESS photometry, precisenear-infrared radial velocities (RV) with the Habitable-zone Planet Finder (HP)Spectrograph, ground-based photometry, and high-contrast imaging. A jointphotometry and RV fit yields a radius $R_p~=~3.37_{-0.20}^{+0.15}\:\mathrm{R_\oplus}$, mass $m_p~=~16.4_{-4.1}^{+4.1}\:\mathrm{M_\oplus}$, anddensity $\rho_p~=~2.32_{-0.37}^{+0.38} \:\mathrm{g cm^{-3}}$ for TOI-2015b,suggesting a likely volatile-rich planet. The young, active host star has arotation period of $P_{\mathrm{rot}}~=~8.7 \pm~0.9~\mathrm{days}$ andassociated rotation-based age estimate of $1.1~\pm~0.1\:\mathrm{Gyr}$. Thoughno other transiting planets are seen in the TESS data, the system shows clearTTVs of super period $P_{\mathrm{sup}}~\approx~430\:\mathrm{days}$ andamplitude $\sim$$100\:\mathrm{minutes}$. After considering multiple likelyperiod ratio models, we show an outer planet candidate near a 2:1 resonance canexplain the observed TTVs while offering a dynamically stable solution.However, other possible two-planet solutions -- including 3:2 and 4:3 resonance-- cannot be conclusively excluded without further observations. Assuming a 2:1resonance in the joint TTV-RV modeling suggests a mass of$m_b~=~13.3_{-4.5}^{+4.7}\:\mathrm{M_\oplus}$ for TOI-2015b and$m_c~=~6.8_{-2.3}^{+3.5}\:\mathrm{M_\oplus}$ for the outer candidate.Additional transit and RV observations will be beneficial to explicitlyidentify the resonance and further characterize the properties of the system.

TOI-2015b: A Warm Neptune with Transit Timing Variations Orbiting an Active mid M Dwarf