The transition radiation tracker (TRT) is the outermost of the three sub-systems of the ATLAS inner detector at the Large Hadron Collider (LHC) at CERN. It is designed to combine the drift tube tracker with the transition radiation detector, providing an important contribution to the charged particles precise momentum measurement and particle (mainly electron) identification. The TRT consists of a barrel section at small pseudorapidity (eta) and two separate end-cap partitions at large eta. The detector performance and its operational conditions were permanently monitored during all commissioning and data-taking stages using various software tools, one of which - TRTViewer - is described in the present paper. The TRTViewer is the dedicated program for monitoring the TRT raw data quality and detector performance at different hardware levels: individual straws, readout chips and electronic boards. The data analysis results can be presented on the event-by-event basis or in the form of color maps representing the operation parameters (efficiencies, timing, occupancy, etc.) according to the real geometrical position of the detector hardware elements. The paper describes the TRTViewer software package as the event displaying tool, raw data processor and histogram and operation parameters presenter, which works with the different sources of input information: raw data files, online monitoring histograms, offline analysis histograms and TRT DAQ Configuration database. The package proved to be one of the main instruments for the fast and effective TRT diagnostics during debugging and operation periods.