Dominant Kitaev interaction and field-induced quantum phase transitions in triangular-lattice <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>KCeSe</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>

Realizing Kitaev interactions on triangular lattices offers a compelling platform for exploring quantum spin liquid physics beyond the conventional honeycomb-lattice framework. Here, we investigate the triangular-lattice antiferromagnet <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>KCeSe</a:mi><a:mn>2</a:mn></a:msub></a:math>, where multiple probes reveal strong magnetic anisotropy, suggesting significant Kitaev physics. Through detailed and combined analysis of magnetization, neutron scattering, and thermodynamic experiments, we identify dominant ferromagnetic Kitaev <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mo>(</b:mo><b:mi>K</b:mi><b:mo>=</b:mo><b:mo>−</b:mo><b:mn>1.82</b:mn><b:mspace width="0.16em"/><b:mi mathvariant="normal">K</b:mi><b:mo>)</b:mo></b:mrow></b:math> and antiferromagnetic Heisenberg <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mo>(</e:mo><e:mi>J</e:mi><e:mo>=</e:mo><e:mn>1.34</e:mn><e:mspace width="0.16em"/><e:mi mathvariant="normal">K</e:mi><e:mo>)</e:mo></e:mrow></e:math> interactions that stabilize a stripe-<h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:mi>y</h:mi><h:mi>z</h:mi></h:mrow></h:math> ordered ground state via an order-by-disorder mechanism. Magnetic fields applied along the Kitaev bond direction induce two phase transitions at 1.67 and 3.8 T, consistent with density matrix renormalization group calculation predictions of a progression from stripe-<i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mi>y</i:mi><i:mi>z</i:mi></i:mrow></i:math> to stripe-canted and spin-polarized phases. Near the 1.67 T quantum critical point, enhanced quantum fluctuations suggest conditions favorable for exotic excitations. These results establish <j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:msub><j:mi>KCeSe</j:mi><j:mn>2</j:mn></j:msub></j:math> as a platform for exploring Kitaev physics on triangular lattices.