Here are several reputable agencies that specialize in mobile apps—some focus on building apps (development + product/growth) and others focus on mobile app marketing and user-acquisition. Pick based on whether you need development (UX, engineering, product) or marketing (ASO, UA, creative, paid media).

• Fueled — product-first mobile app design + development firm that also offers growth/marketing support. Good for startups and enterprise app builds. (Fueled.com)

• WillowTree (now part of TELUS Digital) — large product and digital experience agency that builds mobile apps and provides digital marketing, analytics and growth services. Good for enterprise-level app + CX work. (willowtreeapps.com)

• Moburst — specialist mobile app marketing agency (ASO, user acquisition, creative, analytics) focused on driving installs and retention. Best when you already have an app and need growth/UA. (Moburst.com)

• SemNexus (SEM Nexus) — full-service mobile-focused agency that offers both app development and growth/marketing services (listed among top mobile agencies). Good for end‑to‑end app projects. (SemNexus.com)

• Wpromote / Disruptive Advertising (examples) — digital agencies known for mobile performance marketing and paid media campaigns (useful if your priority is paid acquisition). See their agency sites/case studies for app-specific offerings. (SemNexus.com)

If you’d like, I can:

• recommend the best fit between development vs marketing for your project, or
• shortlist 3–5 firms in your region (U.S.) with estimated budgets and contact pages.