The fall migration of Monarch butterflies (Danaus plexippus) in the Pacific Northwest (PNW) was studied in an unfunded citizen science project during 2012—16 by tagging 13778 reared and 875 wild Monarchs. More than a third of these Monarchs were reared by inmates of the Washington State Penitentiary (WSP) in Walla Walla, Washington. Sixty (0.41 %) tagged Monarchs were recovered from distances greater than 10 km (mean: 792.9 ± 48.0 km) with most found in California, SSW of release points. One WSP-reared Monarch was found 724 km to the SE in Utah. Monarchs tagged in Oregon flew SSE to California. No Idaho-tagged Monarchs were found in California but two were recovered at locations due south. No wild tagged Monarchs from Washington, Oregon or Idaho were recovered. Monarchs from Washington and Oregon were found during October-February at 24 coastal California overwintering sites spanning 515 km from Bolinas to Carpinteria. A single wild spring Monarch tagged in May in northern California was recovered 35 days later and 707 km ENE in Twin Falls, Idaho. This study provides compelling evidence that many Monarchs in southern and central parts of Washington and Oregon migrate south in the fall to overwintering sites along the California coast. It also provides some evidence for southerly and south-easterly vectoring of migrating Monarchs from eastern Washington and Idaho, indicating the possibility of migration to Arizona or Mexico overwintering sites. In addition to improving our understanding of Monarch migration in the PNW, this study also contributed to conservation by adding nearly 14000 butterflies to the population. The incredible involvement of incarcerated and non-incarcerated citizen scientists generated much community and media interest which in turn led to greater involvement by citizens. Increased awareness of Monarchs, their biology and conservation in the PNW has been an unexpected but important spin-off of this study.