$^{53}$University of Tsukuba, Tsukuba, Ibaraki 305, Japan $^{54}$Tufts University, Medford, Massachusetts 02155, USA $^{55}$University of Virginia, Charlottesville, VA 22906, USA $^{56}$Waseda University, Tokyo 169, Japan $^{57}$Wayne State University, Detroit, Michigan 48201, USA $^{58}$University of Wisconsin, Madison, Wisconsin 53706, USA $^{59}$Yale University, New Haven, Connecticut 06520, USA $^{60}$Istituto Nazionale di Fisica Nucleare Pisa, $^{bb}$University of Pisa, $^{cc}$University of Siena and $^{dd}$Scuola Normale Superiore, I-56127 Pisa, Italy (Dated: May 29, 2018, version 1.1) We present a search for the rare radiative decay W ± → π $^{±}$γ using data corresponding to an inte- grated luminosity of 4.3 fb − 1 of proton-antiproton collisions at a center of mass energy of 1.96 TeV collected by the CDF experiment at Fermilab. As no statistically significant signal is observed, we set a 95% confidence level upper limit on the relative branching fraction Γ( W ± → π $^{±}$γ ) / Γ( W ± → e $^{±}$ν ) at 6 . 4 × 10 − $^{5}$, a factor of 10 improvement over the previous limit. PACS numbers: 14.70.Fm, 12.15.Ji, 13.38.Be Since their discovery by the UA1 [1] and UA2 [2] ex- $^{∗}$Deceased $^{†}$With visitors from $^{a}$University of Massachusetts Amherst, Amherst, Massachusetts 01003, $^{b}$Istituto Nazionale di Fisica Nu- cleare, Sezione di Cagliari, 09042 Monserrato (Cagliari), Italy, $^{c}$University of California Irvine, Irvine, CA 92697, $^{d}$University of California Santa Barbara, Santa Barbara, CA 93106 $^{e}$University of California Santa Cruz, Santa Cruz, CA 95064, $^{f}$CERN,CH- 1211 Geneva, Switzerland, $^{g}$Cornell University, Ithaca, NY 14853, $^{h}$University of Cyprus, Nicosia CY-1678, Cyprus, $^{i}$University Col- lege Dublin, Dublin 4, Ireland, $^{j}$University of Fukui, Fukui City, Fukui Prefecture, Japan 910-0017, $^{k}$Universidad Iberoamericana, Mexico D.F., Mexico, $^{l}$Iowa State University, Ames, IA 50011, $^{m}$University of Iowa, Iowa City, IA 52242, $^{n}$Kinki University, Higashi-Osaka City, Japan 577-8502, $^{o}$Kansas State University, Manhattan, KS 66506, $^{p}$University of Manchester, Manchester M13 7