Is the word décima the same in Brazilian and European Portuguese?

Yes

The word "décima" is identical in meaning, grammar, spelling, and usage in both Brazilian and Continental Portuguese. The only difference between the two dialects is pronunciation. In Brazilian Portuguese, the vowels are generally more "open" and the rhythm of speech is more syllable-timed. In Continental Portuguese, the unstressed vowels are more "closed" or often reduced/elided, and the rhythm is more stress-timed.

Brazilian Portuguese Examples

1. Ela ficou na décima posição. (She finished in tenth position.)
2. Esta é a décima vez que eu te aviso. (This is the tenth time I am warning you.)
3. A décima parada do ônibus está aqui. (The tenth bus stop is here.)
4. A décima parte do valor foi paga hoje. (The tenth part of the amount was paid today.)
5. Estamos entrando na décima semana de aulas. (We are entering the tenth week of classes.)

Continental Portuguese Examples

1. Ela ficou na décima posição. (She finished in tenth position.)
2. Esta é a décima vez que te aviso. (This is the tenth time I am warning you.)
3. A décima paragem do autocarro está aqui. (The tenth bus stop is here.)
4. A décima parte do valor foi paga hoje. (The tenth part of the amount was paid today.)
5. Estamos a entrar na décima semana de aulas. (We are entering the tenth week of classes.)