We present a new webservice, Syngine, running at the Incorporated Research Institutions for Seismology Data Management Center (IRIS‐DMC), that offers on‐demand and custom‐tailored seismograms served over HTTP. The free service produces full seismic waveforms, including effects like attenuation and anisotropy, that are calculated in commonly used spherically symmetric Earth models (preliminary reference Earth model [PREM], ak135‐f, IASP91). Users can freely adjust sources and receivers, retrieve seismograms from finite sources, convolve with arbitrary source time functions, and download Green’s functions suitable for moment tensor inversions. Syngine extracts and processes seismograms in as fast as fractions of a second, making it suitable for applications demanding short iteration times and a large number of waveforms. For the first time, researchers without large computational resources or specialized knowledge can easily access high‐quality, custom, broadband seismograms. In this article, we present the rational and basic principles of our method, including its limitations. Additionally, we demonstrate the features of Syngine and the included Earth models, showcase several applications, and discuss future possibilities.