Note that immunologists typically use the word 'development' to refer to cells of B or T cell lineages undergoing the process that GO describes as 'cell differentiation'.

Any process that stops, prevents, or reduces the frequency, rate or extent of T cell differentiation.

downregulation of T cell differentiation

negative regulation of T lymphocyte differentiation

negative regulation of T-lymphocyte differentiation

down regulation of T cell differentiation

down-regulation of T cell differentiation

negative regulation of T-cell differentiation

inhibition of T cell differentiation

biological_process

negative regulation of T cell development

GO:0045581