The pair's trip comes despite warnings from the UK's Foreign, Commonwealth and Development Office (FCDO) not to travel to the country.
There are fears that Russia, which currently has over 100,000 troops near its border with Ukraine, could invade the country imminently.
The FCDO has told all British nationals to leave the country and on Saturday (February 19) said: "British nationals should leave while commercial options remain.
"In the event of a military incursion, it is likely that commercial routes out of Ukraine will be severely disrupted and roads across Ukraine could be closed."
However, the pair have taken the decision to travel in a personal capacity and they form part of an anti-war delegation that includes trade union and cross-party representatives.
During their trip they will meet Ukrainian workers, LGBTQ+ people, ethnic minorities, and human rights activists.
Mr Antoniw, MS for Pontypridd and the Welsh Government's Counsel General, was born in the country and has regularly visited the country at the height of political tension, including during the Maidan protests in 2014.
In a tweet, Mr Antoniw said: "Arrived in Kyiv. Peaceful, calm and normal with Adam Price ready to meet trade unions and civic society. Solidarity with Ukrainian people against Putin's imperialism."
Speaking ahead of the visit, Plaid Cymru leader Adam Price said: "The more the Ukrainian people are threatened by Russian aggression and imperialism the more urgent it becomes for socialists, democrats and internationalist to stand shoulder to shoulder in solidarity with them - in defence of their right to national self-determination and in defiance of Putin’s warmongering."
Boris Johnson has warned that Russia appears to be planning the "biggest war in Europe since 1945", after Ukraine President Volodymyr Zelensky asked Vladimir Putin for a meeting to discuss his demands.
The prime minister told world leaders on Saturday at the Munich Security Conference that a Russian invasion of Ukraine would spark a “generation of bloodshed and misery" - while President Putin oversaw nuclear drills alongside Belarusian President Alexander Lukashenko.