That's a significant increase in usage compared with the same time last year, when just 30% of 18-24-year-olds were using Snapchat.
Use of the mobile messaging app has also spiked with older Millennials, age 25-34, with 20% now using it—up from around 8% last year.
Usage among non-Millennials, adults age 35+, has not increased at the same rate, remaining under 10% for the past year.
Snapchat's combined penetration for both Millennial segments (18-24 years old and 25-34 years old) is now at 32.9%, making it one of the most used social apps with the age group, the analysis found.
Only Facebook (75.6% penetration) and Instagram (43.1%) have more Millennial smartphone users than Snapchat.
The rest of the major social apps now all have fewer Millennial users than Snapchat, including Twitter (23.8%), Pinterest (17.9%), Google+ (18%), Vine (10.7%), and Tumblr (6.3%).