The envelope elimination and restoration (EER) approach developed by Kahn and its modern derivative, a polar transmitter, are well recognized as highly efficient amplification schemes. This paper presents an EER/polar dual-mode transmitter using injection-locked oscillators (ILOs). For an EER operation, an ILO that is combined with a mixer and a low-pass filter in the proposed architecture extracts an envelope signal and a phase-modulated RF signal from an input RF signal with complex modulation. The phase-modulated RF signal is then amplified efficiently by a class-E power amplifier (PA). Additionally, the envelope signal is reconstructed at the PA output by modulating the supply voltage of the PA. For a polar operation, the necessary phase modulation and envelope modulation are generated in the digital domain instead. Moreover, the phase-modulated RF signal is injected into an ILO and the ILO output is amplified by a class-E PA while supply-modulated with the envelope signal. For wideband code-division multiple access (WCDMA) signals, the implemented prototype transmitter achieves a 20.8-dB gain and 44% power-added efficiency (PAE) at an average output power of 25.8 dBm. For enhanced data rates for GSM evolution signals, it delivers 26.6 dB of gain and 48.7% PAE at an average output power of 26.6 dBm. Furthermore, to compensate for the AM–AM and AM–PM distortions of the PA, the linearity is improved using static digital predistortion. Experimental results demonstrate that the proposed EER/polar transmitter using ILOs has a high gain, high efficiency, and good linearity.