Abstract: We propose Long-LRM, a feed-forward 3D Gaussian reconstruction model for instant, high-resolution, 360$^\circ$ wide-coverage, scene-level reconstruction. Specifically, it takes in 32 input images at a resolution of $960\times 540$ and produces the Gaussian reconstruction in just 1 second on a single A100 GPU. To handle the long sequence of **250K** tokens brought by the large input size, Long-LRM features a mixture of the recent Mamba2 blocks and the classical transformer blocks, enhanced by a light-weight token merging module and Gaussian pruning steps that balance between quality and efficiency. We evaluate Long-LRM on the large-scale DL3DV benchmark and Tanks&Temples, demonstrating reconstruction quality comparable to the optimization-based methods while achieving an **800**$\times$ speedup w.r.t. the optimization-based approaches and an input size at least **60**$\times$ larger than the previous feed-forward approaches. We conduct extensive ablation studies on our model design choices for both rendering quality and computation efficiency. We also explore Long-LRM's compatibility with other Gaussian variants such as 2D GS, which enhances Long-LRM's ability in geometry reconstruction. Project page: https://longgggglrm.github.io