Krejcikova and Ostapenko are both former winners of the French Open – in 2021 and 2017, respectively – while Ostapenko has also been a semi-finalist at Wimbledon.
They will be joined at Edgbaston Priory Club from June 17-25 by defending champion Beatriz Haddad Maia and last year's losing finalist Shuai Zhang.
Tournament director Patrick Hughesman said: “All but one of the main draw direct acceptances for this year’s tournament are ranked in the WTA top 50, illustrating the strength and depth of the field the event has attracted this year.
"This is an amazing opportunity for the people of Birmingham and beyond to experience world-class tennis and we look forward to welcoming spectators and players to the Rothesay Classic next month.”
Emma Raducanu had been scheduled to take part this year before undergoing multiple surgeries in a bid to get to the bottom of the injury problems that have affected her since winning the US Open in 2021.
But nine of the world’s top 30 women are included on the entry list announced, including former US Open finalist Madison Keys, Magda Linette, Anastasia Potapova, Anhelina Kalinia and Elise Mertens.
Keys lifted the Maud Watson Trophy at Edgbaston Priory Club in 2016, while previous Wimbledon champions Ash Barty, Petra Kvitova and Angelique Kerber are also among the recent winners of the tournament.
Legends of the game Billie Jean King, Pam Shriver and Martina Navratilova have also previously won the tournament – one of the key Wimbledon tune-up events on the women's calendar.
Ajla Tomljanovic is among the other players listed. She has reached the quarter-finals at Wimbledon in each of the last two years and was heavily featured in the Netflix docu-series Break Point.
Sorana Cirstea, Camila Giorgi, Marta Kostyuk and Alycia Parks are among those who have also been announced.
Four wild card places remain in the draw, which will be announced in due course.
Qualifying for the event takes place on June 17-18, with main draw action starting on June 19.