Every day, about 200 million people go on Twitter to share their updates, talk about their interests, and connect with others from all over the world. Although not as popular as platforms like Facebook or Instagram with billions of users, Twitter still has a loyal audience of those that log in on a regular basis. Users use this platform to spread large amounts of information on very diverse topics. For this reason, Twitter is a good place to observe citizen concerns about environmental issues, including marine ones. To determine who, where, when, and for what reason marine plastic pollution is discussed, a data visualization dashboard called Twilitter has been developed. The analysis of 270 K tweets in 12 different languages posted during two years shows, among other things, that London is the city from where this subject is tweeted the most, that World Ocean Day is the most celebrated date and that activity increases after the dissemination of news about relevant or impacting scientific advances. Knowing these data can be vitally important in aligning the messages of the institutions responsible for environmental protection with the interests and way of expressing themselves of the general public.