Rio de Janeiro has a tropical climate and light clothing is quite enough all the year round without packing any cold weather gear. The temperature in the city is 20 degrees on average, reaching 40+ degrees Celsius in hot, humid and blazing high summer months (i.e. December and January) while winters are a little bit rainy and cooler although still comfortably warm.
As a whole the residents of Rio de Janeiro dress fairly informally and casually. Their clothes are simple but with style. Good casual clothes are fine for even the best restaurants and a night out.
Usually natives and tourists alike wear T-shirts, shorts, Havaianas (the Brazilian version of flip flops) and non-slip sandals and shoes (sidewalks can be slippery when it rains).
In winter months something warmer is needed, especially at night. A good choice is a long sleeve shirt, layer clothes and a light jacket.
A one-piece swim suit is always a distinguishing feature of tourists because native girls in Rio don’t sunbathe topless. Bikini style is still alive.
Brazilian Sarong or “Kanga” is more common than a beach towel. Flip flops are necessary for walks along the beach.