According to Lattham & Matthews, this was either Theatrum civitatum … Italiae or Urbium praecipuarum mundi theatrum quintum, two out of six volumes of engravings published in Cologne, 1572-1618. Pepys seems to have confused the titles.
Michael Robinson • Link
Pepys might have been correct,
another possibility is:-
Novum ac magnum theatrum urbium Belgicae ...
Amsterdam: Jan Blaeu, 1649. 2 vols., fo.
This would be consistent with viewing the "Flandria Illustrata" at the same time.
Plates from Blaeu's 'Theatrum Urbium'
Chart showing the number of references in each month of the diary’s entries.