Clifton A. Pemble -- President and Chief Executive Officer
Thank you, Teri, and good morning, everyone. I'd like to begin by mentioning a couple of important milestones we recently celebrated. During the third quarter, we shipped our 200 millionth product, which is a testament to our ability to design, manufacture and sell unique applications of technology for active lifestyles. Equally is exciting, we started production in our new aviation manufacturing facility located in Olathe, Kansas. This new facility more than doubles our production capacity, allowing us to serve our growing aviation business for many years to come.
Moving now to the quarterly results, earlier today, Garmin reported strong third quarter consolidated revenue of $810 million, up 8% over the prior year. Marine, Aviation, Fitness and Outdoor collectively increased 16% year-over-year and contributed 80% of total revenues. Gross margin improved to 59.4% compared to the prior year. Operating income improved to $196 million, up 13% over the prior year. This resulted in GAAP EPS of $0.97 and pro forma EPS of $1 in the quarter. We are pleased with our performance in the first three quarters of 2018 and these strong results give us confidence to raise our full-year EPS guidance. Doug will discuss our financial results in greater detail in a few minutes, but first I'd like to provide a few brief remarks on the performance of our business segments.
Starting with the Marine segment, revenue increased 28% as we saw strong sales continue well into the summer boating season. Approximately half of the growth was organic while the other half came from acquisitions. Gross and operating margins were 59% and 14%, respectively. We recently announced our GPSMAP 8600 series of chartplotters. This is the first product line to use on new G3 maps, which combine the best of Garmin and Navionics content. We've been very intentional about investing in our Marine segment and the industry is taking notice. For the fourth consecutive year, we were recognized by the National Marine Electronics Association as Manufacturer of the Year and Panoptix LiveScope won the prestigious Technology Award. We were also recognized as one of the top 10 most innovative marine companies in 2018 by Soundings Trade Only, which is a B2B news and information provider for the recreational boating industry. It's an honor to be recognized by the marine industry and we will continue to invest in this segment to maximize its potential.
Turning next to Aviation, revenue increased 17%, driven by broad-based growth within this segment. Gross and operating margins increased to 76% and 35%, respectively, resulting in operating income growth of 49% over the prior year. During the quarter, we completed the acquisition of FltPlan.com and have begun integrating these new services in the Garmin's existing apps. Also, we recently announced that our ADS-B solution was selected by Gulfstream for the G280 aircraft. Finally, we announced a teaming agreement with Bell to supply avionics for on-demand mobility vehicles. While this project is in its early stages, it's an important first step toward creating a viable urban air transport system.
Turning next to the Fitness segment, revenue increased 14% primarily driven by growth of our wearable products. Gross and operating margins were 54% and 20%, respectively, and operating income grew 12% over the prior year. During the third quarter, we launched the vivosmart 4, a slim smart activity tracker that includes a pulse ox sensor. In addition to providing blood oxygen saturation levels, this device also provides users with advanced sleep monitoring and the new Body Battery feature that helps individuals understand and manage their energy levels throughout the day. We also added Disney Princess and Marvel's Spider-Man bands to our popular vivofit jr. product line along with new mobile app adventures.
Turning next to the Outdoor segment, revenue increased 13% on a year-over-year basis, driven primarily by growth in wearables. Gross and operating margins improved year-over-year to 65% and 38%, respectively, resulting in operating income growth of 16%. We recently announced the integration of Spotify for the fenix 5 Plus series and just this morning the Forerunner 645 music was added to the list of Spotify compatible wearables. This gives our customers the ability to download Spotify playlists to the watch via the Spotify app, which is available from the Connect IQ store. The app is already proving to be very popular with customers. During its first full day of availability, Spotify set a record for the most downloads of a new app from our Connect IQ store. Finally, we recently announced Instinct, a rugged and reliable GPS smartwatch designed to expand the market for outdoor wearables.
Looking finally at the Auto segment, revenues decreased 16% due to the ongoing decline of the PND market. Gross and operating margins declined year-over-year to 43% and 9%, respectively. Our global market share position in the PND category remains very strong. We recently announced that we've been selected by the Chinese auto group, Geely, to provide camera and driving recorded systems beginning in model year 2020. This award demonstrates the progress we are making as a Tier 1 auto supplier.
In summary, we are pleased with our results in the first three quarters of 2018. In light of the strong third quarter results, we are making some adjustments to our guidance. We anticipate our fourth quarter revenue to be relatively flat on a year-over-year basis with full-year revenue of approximately $3.3 billion and a gross margin of 58.5%. We are raising our full-year operating margin to approximately 22% and lowering our full-year pro forma effective tax rate to approximately 16%, resulting in pro forma earnings per share of approximately $3.45. That concludes my remarks.
Garmin Ltd. (GRMN) Q3 2018 Earnings Conference Call Transcript -- The Motley Fool
