The 16-year-old arrived in Portugal onboard a sailboat after 20 days at sea.
She made the three-week journey on a catamaran with a British navigator and an Australian sailing couple after the summit was moved from Chile to Madrid because of unrest in its original host country.
She wants senior officials in Madrid to "finally understand the urgency" of climate change and co-operate internationally.
She arrived at 12:45 GMT to a welcome ceremony attended by Lisbon mayor Fernando Medina. "We are all looking forward to see you there!"
After arriving in Lisbon, Greta said that people and adults continue to underestimate angry kids.
COP25 was originally planned to take place in Chile, but was changed to Madrid because of protests and instability in the South American nation.
Thunberg has refused to travel by plane due to their harmful emissions and has even spoken out against their use and encouraged people to forego using them and thus chose to cross the Atlantic via sailboat.
The climate summit, which opened yesterday and runs to 13 December, brings together representatives of some 200 signatory nations to the 2015 Paris accord.
"Many people say that nothing will come out of the COP25. We are angry and frustrated, but for good reason", she added.
Despite the grim statistics, young delegates said they were inspired by Ms Thunberg's determination to minimise her own carbon footprint by twice braving the Atlantic instead of flying - which they saw as a symbol of resourcefulness and defiance.
After using her appearance in NY to denounce governments for failing to act with sufficient urgency on the environment, Ms Thunberg attended weekly green protests in several United States cities before embarking on her long trip back to Europe.