Principal Machine Learning Engineer - League of Legends

At Riot, we are investing into League of Legends to grow our beloved game for generations to come. With Summoner’s Rift, ARAM Mayhem, and plenty of other gameplay experiences in the works, we are focused on improving our new player experience across many vectors, including our Bots. We want players to be able to learn how to play in fun, dynamic environments in which human-like Bots surprise, delight, and challenge them to learn the game.

As a Principal Machine Learning Engineer, you will unlock better Bots for League of Legends at a pivotal time in the game’s history. You will be forming and testing various ML and AI approaches to training bots that can play with and against human players of varying skill and experience levels. This is a key area of our product strategy and you will be at the forefront of us improving the player experience for millions of players across the globe. Reporting directly into the game team, you will collaborate with game engineers, UX designers, and Insights analysts to test, optimize, and deploy your bot approaches utilizing live data and experimentation. You could also be involved in building a variety of ML applications for League including but not limited to content generation, artist tools, and design tools.

• Lead the modeling strategy for AI Bots on League of Legends, focusing on training agents that can understand game state, make decisions, and create compelling player experiences across a variety of game modes and skill levels.
• Develop predictive features and signals from gameplay telemetry, unstructured game data, and simulation outputs, ensuring quality, interpretability, and reliability.
• Design and implement ML systems using methods including reinforcement learning and imitation learning, on-/off-policy algorithms, policy gradient methods, behavior shaping, and hybrid systems that combine learned policies with rule-based or scripted components.
• Define evaluation frameworks for game AI that balance generalizable approaches with genre-specific metrics, adapting methods for the needs of each game mode.
• Ensure the safety, fairness, and trustworthiness of autonomous agents operating in live player environments.
• Collaborate with game and platform engineers, along with UX teams, to integrate models into production in ways that enhance player experience and maintain operational reliability.
• Contribute to the AI community at Riot, sharing methods and learnings with other MLEs across the company.

• Extensive experience (8+ years) delivering ML systems in production, including reinforcement learning, imitation learning, or simulation-based training in rich, interactive environments such as game worlds or multi-agent simulations.
• Proven ability to design modeling strategies and architectures adopted across multiple games or interactive products.
• Expertise working with gameplay telemetry, simulation data, or other complex interactive environments for feature engineering and training. 
• Game engineering/C++ experience to write telemetry hooks is a plus.
• Strong track record building and optimizing agent-based systems or world models for dynamic, player-facing environments.
• Hands-on experience with relevant ML methods including reinforcement learning and imitation learning (such as behavior cloning and inverse reinforcement learning), on-/off-policy algorithms, policy gradient methods, behavior shaping, and hybrid systems that combine learned policies with rule-based or scripted components.
• Mastery of experiment design, model evaluation, and optimization for autonomous agents.
• Track record of incorporating human considerations into AI applications, such as responsible AI safety and UX best practices.
• League of Legends game context and/or demonstrated ability to learn competitive game systems at a rapid rate..
• Familiarity with integrating ML-driven agents into live game environments with game and platform engineers.

For this role, you'll find success through craft expertise, a collaborative spirit, and decision-making that prioritizes the delight of players. We will be looking at your past studies, experience, and your personal relationship with games. If you embody player empathy and care about players' experiences, this could be your role!

Riot focuses on work/life balance, shown by our open paid time off policy and other perks such as flexible work schedules. We offer medical, dental, and life insurance, parental leave for you, your spouse/domestic partner, and children, and a 401k with company match. Check out our benefits pages for more information.

At Riot Games, we put players first. That mission drives every decision in our quest to create games and experiences that make it better to be a player. Whether you’re working directly on a new player-facing experience or you’re supporting the company as a whole, everyone at Riot is part of our mission. And just like in our games, we’re better when we work together. Our goal is to create collaborative teams where you are empowered to bring your unique perspective everyday. If that sounds like the kind of place you want to work, we’re looking forward to your application.

It’s our policy to provide equal employment opportunity for all applicants and members of Riot Games, Inc. Riot Games makes reasonable accommodations for handicapped and disabled Rioters and does not unlawfully discriminate on the basis of race, color, religion, sex, sexual orientation, gender identity or expression, national origin, age, handicap, veteran status, marital status, criminal history, or any other category protected by applicable federal and state law. We consider for employment all qualified applicants, including those with criminal histories, in a manner consistent with applicable federal, state and local law, including the California Fair Chance Act, the City of Los Angeles Fair Chance Initiative for Hiring Ordinance, the Los Angeles County Fair Chance Ordinance for Employers, the San Francisco Fair Chance Ordinance, and the Washington Fair Chance Act.

Per the Los Angeles County Fair Chance Ordinance, the following core duties may create a basis for disqualifying candidates with relevant criminal histories:

• Safeguarding confidential and sensitive Company data
• Communication with others, including Rioters and third parties such as vendors, and/or players, including minors
• Accessing Company assets, secure digital systems, and networks
• Ensuring a safe interactive environment for players and other Rioters

These duties are directly related to essential operations, safety, trust, and compliance obligations within our organization. Please note that job duties may evolve based on business needs and additional responsibilities may be assigned as necessary to maintain operational efficiency and security.