Cleaning mutualisms are key ecological components in coral reef ecosystems and crucial drivers of marine biodiversity and abundance. These mutualisms are one of the most common interspecific interactions worldwide, which involves a cleaner organism that removes and eats ectoparasites from their so-called “clients” (usually larger reef fish) in complex and cognitive demanding cooperative behaviors that can involve tactile stimulation and partner control. Nonetheless, until now, there is no knowledge on the potential effects of climate change on such mutualistic interactions. Here we investigated, for the first time, how cleaning mutualisms may respond to future chronic conditions of ocean acidification (Δ pH=0.4) and warming (+4 °C). To accomplish such goal, we used two cleaner-client pairs from two geographically-different coral reef systems, namely cleaner wrasses Labroides dimidiatus and their clients Naso elegans from the Indo-Pacific, and cleaner gobies Elacatinus oceanops and their clients Haemulon flavolineatum from the Caribbean Sea. After 45 days of acclimation (in separated individual tanks), pairs of cleaners and clients were allowed to interact in an observation tank for one hour. We measured inspection (frequency, number of bites, duration), tactile stimulation, (frequency, duration), honesty (jolt frequency, chases) and motivation (who started the interaction) and compared to both cleaner and client physiological state. As well as describing how this comprehensive set of behavioural responses to the conditions of the ocean of tomorrow, we discuss the related jeopardies for coral reef ecosystem’s health and functioning.