Cameron Lemon, Ariel Goobar, Joel Johansson, Edvard Mörtsell, Steve Schulze, Igor Andreoni, Aleksandra Bochenek, Seán J. Brennan, Malte Busmann, Michael Coughlin, Kaustav K. Das, Suhail Dhawan, Christoffer Fremling, Anjasha Gangopadhyay, Daniel Gruen, Xander J. Hall, Anna Y. Q. Ho, Mansi M. Kasliwal, Daniel A. Perley, Mickael Rigault, Genevieve Schroeder, Mathew Smith, Jesper Sollerman, Jean J. Somalwar, Robert Stein, Stephen Thorp, Alice Townsend, Jacob L. Wise, Lin Yan, Nikki Arendse, Eric C. Bellm, Tracy X. Chen, Andrew Drake, Frank J. Masci, Josiah Purdum, Roger Smith, Jason T. Hinkle, T. Emil Rivera-Thorsen, Benjamin J. Shappee, Michael A. Tucker, Jessica Aguilar, Steven Ahlen, Greg Aldering, Segev Benzvi, Davide Bianchi, David Brooks, Todd Claybaugh, Axel de la Macorra, John Della Costa, Arjun Dey, Peter Doel, Brenna Flaugher, Andreu Font-Ribera, Jaime E. Forero-Romero, Enrique Gaztañaga, Satya Gontcho A. Gontcho, Gaston Gutierrez, Dragan Huterer, Mustapha Ishak, Jorge Jimenez, Dick Joyce, Stephanie Juneau, Robert Kehoe, Alex G. Kim, David Kirkby, Theodore Kisner, Anthony Kremin, Ofer Lahav, Martin Landriau, Laurent Le Guillou, Michael E. Levi, Marc Manera, Aaron Meisner, Ramon Miquel, John Moustakas, Seshadri Nadathur, Brendan O'Connor, Nathalie Palanque-Delabrouille, Antonella Palmese, Will J. Percival, Ignasi Pérez-Ràfols, Claire Poppett, Francisco Prada, Graziano Rossi, Eusebio Sanchez, David Schlegel, Michael Schubnell, Arman Shafieloo, Joseph Silber, David Sprayberry, Gregory Tarlé, Benjamin A. Weaver, Hu Zou

We present the discovery of SN 2025mkn, a gravitationally lensed Type II supernova. First detected as a blue transient in ZTF, 0.83$^{\prime\prime}$ from a $z=0.42$ elliptical galaxy, follow-up SNIFS/UH2.2m and LRIS/Keck spectra revealed absorption lines at $z=1.371$. Later JWST NIRCam imaging shows that the bright transient is a close pair of point sources separated by $\sim 0.07^{\prime\prime}$, and a 30 times fainter counterimage opposite the lens, for which NIRSpec reveals strong H$α$ emission also at $z=1.371$. The light curves and spectra are consistent with the Type II supernova source being magnified $\gtrsim 100$ times, with $\sim 250$ required to reconcile its luminosity with that of nearby events such as SN 2023ixf. Lens models are consistent with such high magnifications, and always show that the faint image arrived first (undetected in earlier ZTF imaging), consistent with the later spectral phase of this fainter image. A fourth image is also predicted and possibly detected in the NIRSpec data. Light-curve-based time-delay measurements are not possible due to the first image being the faintest; however, the resolved NIRSpec spectra offer a future opportunity for time-delay cosmography through supernova phase measurements.